diff --git a/.ameba.yml b/.ameba.yml
new file mode 100644
index 0000000..399b108
--- /dev/null
+++ b/.ameba.yml
@@ -0,0 +1,3 @@
+Lint/NotNil:
+  Excluded:
+    - spec/backtracer/backtrace/frame/parser_spec.cr
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..ac9ac2f
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,46 @@
+name: CI
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: "0 3 * * 1" # Every monday at 3 AM
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+        crystal: [latest, nightly]
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Install Crystal
+        uses: oprypin/install-crystal@v1
+        with:
+          crystal: ${{ matrix.crystal }}
+
+      - name: Download source
+        uses: actions/checkout@v2
+
+      - name: Install dependencies
+        run: shards install
+        env:
+          SHARDS_OPTS: --ignore-crystal-version
+
+      - name: Run specs
+        run: |
+          crystal spec
+          crystal spec --no-debug
+
+      - name: Run specs (release)
+        run: |
+          crystal spec --release
+          crystal spec --release --no-debug
+
+      - name: Check formatting
+        run: crystal tool format --check
+
+      - name: Run ameba linter
+        run: bin/ameba
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index 7521934..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,20 +0,0 @@
-language: crystal
-
-crystal:
-  - latest
-  - nightly
-
-jobs:
-  allow_failures:
-    - crystal: nightly
-
-install:
-  - shards install
-
-script:
-  - crystal spec
-  - crystal spec --no-debug
-  - crystal spec --release
-  - crystal spec --release --no-debug
-  - crystal tool format --check
-  - bin/ameba
diff --git a/README.md b/README.md
index 03ca6ae..8d17ade 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-# backtracer.cr [![Build Status](https://travis-ci.com/Sija/backtracer.cr.svg?branch=master)](https://travis-ci.com/Sija/backtracer.cr) [![Releases](https://img.shields.io/github/release/Sija/backtracer.cr.svg)](https://github.com/Sija/backtracer.cr/releases) [![License](https://img.shields.io/github/license/Sija/backtracer.cr.svg)](https://github.com/Sija/backtracer.cr/blob/master/LICENSE)
+# backtracer.cr [![CI](https://github.com/Sija/backtracer.cr/actions/workflows/ci.yml/badge.svg)](https://github.com/Sija/backtracer.cr/actions/workflows/ci.yml) [![Releases](https://img.shields.io/github/release/Sija/backtracer.cr.svg)](https://github.com/Sija/backtracer.cr/releases) [![License](https://img.shields.io/github/license/Sija/backtracer.cr.svg)](https://github.com/Sija/backtracer.cr/blob/master/LICENSE)
 
 Crystal shard aiming to assist with parsing backtraces into a structured form.
 
diff --git a/shard.yml b/shard.yml
index 20cf883..96ff072 100644
--- a/shard.yml
+++ b/shard.yml
@@ -1,5 +1,5 @@
 name: backtracer
-version: 1.0.0
+version: 1.2.2
 
 authors:
   - Sijawusz Pur Rahnama <sija@sija.pl>
@@ -7,7 +7,7 @@ authors:
 development_dependencies:
   ameba:
     github: crystal-ameba/ameba
-    version: ~> 0.13.0
+    version: ~> 1.5.0
 
 crystal: ">= 0.35.0"
 
diff --git a/spec/backtracer/backtrace/frame/context_spec.cr b/spec/backtracer/backtrace/frame/context_spec.cr
new file mode 100644
index 0000000..b6fc3aa
--- /dev/null
+++ b/spec/backtracer/backtrace/frame/context_spec.cr
@@ -0,0 +1,60 @@
+require "../../../spec_helper"
+
+def with_foo_context(&)
+  yield Backtracer::Backtrace::Frame::Context.new(
+    lineno: 10,
+    pre: %w[foo bar baz],
+    line: "violent offender!",
+    post: %w[boo far faz],
+  )
+end
+
+describe Backtracer::Backtrace::Frame::Context do
+  describe ".to_a" do
+    it "works with empty #pre and #post" do
+      context = Backtracer::Backtrace::Frame::Context.new(
+        lineno: 1,
+        pre: %w[],
+        line: "violent offender!",
+        post: %w[],
+      )
+      context.to_a.should eq(["violent offender!"])
+    end
+
+    it "returns array with #pre, #line and #post strings" do
+      with_foo_context do |context|
+        context.to_a.should eq([
+          "foo", "bar", "baz",
+          "violent offender!",
+          "boo", "far", "faz",
+        ])
+      end
+    end
+  end
+
+  describe ".to_h" do
+    it "works with empty #pre and #post" do
+      context = Backtracer::Backtrace::Frame::Context.new(
+        lineno: 1,
+        pre: %w[],
+        line: "violent offender!",
+        post: %w[],
+      )
+      context.to_h.should eq({1 => "violent offender!"})
+    end
+
+    it "returns hash with #pre, #line and #post strings" do
+      with_foo_context do |context|
+        context.to_h.should eq({
+           7 => "foo",
+           8 => "bar",
+           9 => "baz",
+          10 => "violent offender!",
+          11 => "boo",
+          12 => "far",
+          13 => "faz",
+        })
+      end
+    end
+  end
+end
diff --git a/spec/backtracer/backtrace/frame/parser_spec.cr b/spec/backtracer/backtrace/frame/parser_spec.cr
new file mode 100644
index 0000000..7df2f9e
--- /dev/null
+++ b/spec/backtracer/backtrace/frame/parser_spec.cr
@@ -0,0 +1,202 @@
+require "../../../spec_helper"
+
+describe Backtracer::Backtrace::Frame::Parser do
+  describe ".parse" do
+    it "fails to parse an empty string" do
+      expect_raises(ArgumentError) { with_frame("", &.itself) }
+    end
+
+    context "when --no-debug flag is set" do
+      it "parses frame with any value as method" do
+        backtrace_line = "__crystal_main"
+
+        with_frame(backtrace_line) do |frame|
+          frame.lineno.should be_nil
+          frame.column.should be_nil
+          frame.method.should eq(backtrace_line)
+          frame.path.should be_nil
+          frame.relative_path.should be_nil
+          frame.under_src_path?.should be_false
+          frame.shard_name.should be_nil
+          frame.in_app?.should be_false
+        end
+      end
+    end
+
+    context "with ~proc signature" do
+      it "parses absolute path outside of src/ dir" do
+        path = "/usr/local/Cellar/crystal/0.27.2/src/fiber.cr"
+        backtrace_line = "~proc2Proc(Fiber, (IO::FileDescriptor | Nil))@#{path}:72"
+
+        with_frame(backtrace_line) do |frame|
+          frame.lineno.should eq(72)
+          frame.column.should be_nil
+          frame.method.should eq("~proc2Proc(Fiber, (IO::FileDescriptor | Nil))")
+          frame.path.should eq(path)
+          frame.absolute_path.should eq(frame.path)
+          frame.relative_path.should be_nil
+          frame.under_src_path?.should be_false
+          frame.shard_name.should be_nil
+          frame.in_app?.should be_false
+        end
+      end
+
+      it "parses relative path inside of lib/ dir" do
+        with_configuration do |configuration|
+          path = "lib/kemal/src/kemal/route.cr"
+          backtrace_line = "~procProc(HTTP::Server::Context, String)@#{path}:11"
+
+          with_frame(backtrace_line) do |frame|
+            frame.lineno.should eq(11)
+            frame.column.should be_nil
+            frame.method.should eq("~procProc(HTTP::Server::Context, String)")
+            frame.path.should eq(path)
+            frame.absolute_path.should eq(
+              File.join(configuration.src_path.not_nil!, path)
+            )
+            frame.relative_path.should eq(frame.path)
+            frame.under_src_path?.should be_false
+            frame.shard_name.should eq("kemal")
+            frame.in_app?.should be_false
+          end
+        end
+      end
+    end
+
+    it "parses absolute path outside of configuration.src_path" do
+      path = "/some/absolute/path/to/foo.cr"
+
+      with_foo_frame(path: path) do |frame|
+        frame.lineno.should eq(1)
+        frame.column.should eq(7)
+        frame.method.should eq("foo_bar?")
+        frame.path.should eq(path)
+        frame.absolute_path.should eq(frame.path)
+        frame.relative_path.should be_nil
+        frame.under_src_path?.should be_false
+        frame.shard_name.should be_nil
+        frame.in_app?.should be_false
+      end
+    end
+
+    context "with in_app? = false" do
+      it "parses absolute path outside of src/ dir" do
+        with_foo_frame(path: "#{__DIR__}/foo.cr") do |frame|
+          frame.lineno.should eq(1)
+          frame.column.should eq(7)
+          frame.method.should eq("foo_bar?")
+          frame.path.should eq("#{__DIR__}/foo.cr")
+          frame.absolute_path.should eq(frame.path)
+          frame.relative_path.should eq("spec/backtracer/backtrace/frame/foo.cr")
+          frame.under_src_path?.should be_true
+          frame.shard_name.should be_nil
+          frame.in_app?.should be_false
+        end
+      end
+
+      it "parses relative path outside of src/ dir" do
+        with_configuration do |configuration|
+          path = "some/relative/path/to/foo.cr"
+
+          with_foo_frame(path: path) do |frame|
+            frame.lineno.should eq(1)
+            frame.column.should eq(7)
+            frame.method.should eq("foo_bar?")
+            frame.path.should eq(path)
+            frame.absolute_path.should eq(
+              File.join(configuration.src_path.not_nil!, path)
+            )
+            frame.relative_path.should eq(frame.path)
+            frame.under_src_path?.should be_false
+            frame.shard_name.should be_nil
+            frame.in_app?.should be_false
+          end
+        end
+      end
+    end
+
+    context "with in_app? = true" do
+      it "parses absolute path inside of src/ dir" do
+        src_path = File.expand_path("../../../../src", __DIR__)
+        path = "#{src_path}/foo.cr"
+
+        with_foo_frame(path: path) do |frame|
+          frame.lineno.should eq(1)
+          frame.column.should eq(7)
+          frame.method.should eq("foo_bar?")
+          frame.path.should eq(path)
+          frame.absolute_path.should eq(frame.path)
+          frame.relative_path.should eq("src/foo.cr")
+          frame.under_src_path?.should be_true
+          frame.shard_name.should be_nil
+          frame.in_app?.should be_true
+        end
+      end
+
+      it "parses relative path inside of src/ dir" do
+        with_configuration do |configuration|
+          path = "src/foo.cr"
+
+          with_foo_frame(path: path) do |frame|
+            frame.lineno.should eq(1)
+            frame.column.should eq(7)
+            frame.method.should eq("foo_bar?")
+            frame.path.should eq(path)
+            frame.absolute_path.should eq(
+              File.join(configuration.src_path.not_nil!, path)
+            )
+            frame.relative_path.should eq(path)
+            frame.under_src_path?.should be_false
+            frame.shard_name.should be_nil
+            frame.in_app?.should be_true
+          end
+        end
+      end
+    end
+
+    context "with shard path" do
+      it "parses absolute path inside of lib/ dir" do
+        lib_path = File.expand_path("../../../../lib/bar", __DIR__)
+        path = "#{lib_path}/src/bar.cr"
+
+        with_foo_frame(path: path) do |frame|
+          frame.lineno.should eq(1)
+          frame.column.should eq(7)
+          frame.method.should eq("foo_bar?")
+          frame.path.should eq(path)
+          frame.absolute_path.should eq(frame.path)
+          frame.relative_path.should eq("lib/bar/src/bar.cr")
+          frame.under_src_path?.should be_true
+          frame.shard_name.should eq "bar"
+          frame.in_app?.should be_false
+        end
+      end
+
+      it "parses relative path inside of lib/ dir" do
+        with_configuration do |configuration|
+          path = "lib/bar/src/bar.cr"
+
+          with_foo_frame(path: path) do |frame|
+            frame.lineno.should eq(1)
+            frame.column.should eq(7)
+            frame.method.should eq("foo_bar?")
+            frame.path.should eq(path)
+            frame.absolute_path.should eq(
+              File.join(configuration.src_path.not_nil!, path)
+            )
+            frame.relative_path.should eq(path)
+            frame.under_src_path?.should be_false
+            frame.shard_name.should eq "bar"
+            frame.in_app?.should be_false
+          end
+        end
+      end
+
+      it "uses only folders for shard names" do
+        with_foo_frame(path: "lib/bar.cr") do |frame|
+          frame.shard_name.should be_nil
+        end
+      end
+    end
+  end
+end
diff --git a/spec/backtracer/backtrace/frame_spec.cr b/spec/backtracer/backtrace/frame_spec.cr
index 5f6032c..53c3766 100644
--- a/spec/backtracer/backtrace/frame_spec.cr
+++ b/spec/backtracer/backtrace/frame_spec.cr
@@ -1,207 +1,6 @@
 require "../../spec_helper"
 
-private def parse_frame(line)
-  Backtracer::Backtrace::Frame::Parser.parse(line)
-end
-
-private def with_frame(method, path = nil, lineno = nil, column = nil)
-  line = String.build do |io|
-    if path
-      io << path
-      io << ':' << lineno if lineno
-      io << ':' << column if column
-      io << " in '" << method << '\''
-    else
-      io << method
-    end
-  end
-  yield parse_frame(line)
-end
-
-private def with_foo_frame(
-  method = "foo_bar?",
-  path = "#{__DIR__}/foo.cr",
-  lineno = 1,
-  column = 7
-)
-  with_frame(method, path, lineno, column) do |frame|
-    yield frame
-  end
-end
-
 describe Backtracer::Backtrace::Frame do
-  describe ".parse" do
-    it "fails to parse an empty string" do
-      expect_raises(ArgumentError) { parse_frame("") }
-    end
-
-    context "when --no-debug flag is set" do
-      it "parses frame with any value as method" do
-        backtrace_line = "__crystal_main"
-
-        with_frame(backtrace_line) do |frame|
-          frame.lineno.should be_nil
-          frame.column.should be_nil
-          frame.method.should eq(backtrace_line)
-          frame.path.should be_nil
-          frame.relative_path.should be_nil
-          frame.under_src_path?.should be_false
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_false
-        end
-      end
-    end
-
-    context "with ~proc signature" do
-      it "parses absolute path outside of src/ dir" do
-        backtrace_line = "~proc2Proc(Fiber, (IO::FileDescriptor | Nil))@/usr/local/Cellar/crystal/0.27.2/src/fiber.cr:72"
-
-        with_frame(backtrace_line) do |frame|
-          frame.lineno.should eq(72)
-          frame.column.should be_nil
-          frame.method.should eq("~proc2Proc(Fiber, (IO::FileDescriptor | Nil))")
-          frame.path.should eq("/usr/local/Cellar/crystal/0.27.2/src/fiber.cr")
-          frame.relative_path.should be_nil
-          frame.under_src_path?.should be_false
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_false
-        end
-      end
-
-      it "parses relative path inside of lib/ dir" do
-        backtrace_line = "~procProc(HTTP::Server::Context, String)@lib/kemal/src/kemal/route.cr:11"
-
-        with_frame(backtrace_line) do |frame|
-          frame.lineno.should eq(11)
-          frame.column.should be_nil
-          frame.method.should eq("~procProc(HTTP::Server::Context, String)")
-          frame.path.should eq("lib/kemal/src/kemal/route.cr")
-          frame.relative_path.should eq("lib/kemal/src/kemal/route.cr")
-          frame.under_src_path?.should be_false
-          frame.shard_name.should eq("kemal")
-          frame.in_app?.should be_false
-        end
-      end
-    end
-
-    it "parses absolute path outside of configuration.src_path" do
-      path = "/some/absolute/path/to/foo.cr"
-
-      with_foo_frame(path: path) do |frame|
-        frame.lineno.should eq(1)
-        frame.column.should eq(7)
-        frame.method.should eq("foo_bar?")
-        frame.path.should eq(path)
-        frame.relative_path.should be_nil
-        frame.under_src_path?.should be_false
-        frame.shard_name.should be_nil
-        frame.in_app?.should be_false
-      end
-    end
-
-    context "with in_app? = false" do
-      it "parses absolute path outside of src/ dir" do
-        with_foo_frame do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq("#{__DIR__}/foo.cr")
-          frame.relative_path.should eq("spec/backtracer/backtrace/foo.cr")
-          frame.under_src_path?.should be_true
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_false
-        end
-      end
-
-      it "parses relative path outside of src/ dir" do
-        path = "some/relative/path/to/foo.cr"
-
-        with_foo_frame(path: path) do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq(path)
-          frame.relative_path.should eq(path)
-          frame.under_src_path?.should be_false
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_false
-        end
-      end
-    end
-
-    context "with in_app? = true" do
-      it "parses absolute path inside of src/ dir" do
-        src_path = File.expand_path("../../../src", __DIR__)
-        path = "#{src_path}/foo.cr"
-
-        with_foo_frame(path: path) do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq(path)
-          frame.relative_path.should eq("src/foo.cr")
-          frame.under_src_path?.should be_true
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_true
-        end
-      end
-
-      it "parses relative path inside of src/ dir" do
-        path = "src/foo.cr"
-
-        with_foo_frame(path: path) do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq(path)
-          frame.relative_path.should eq(path)
-          frame.under_src_path?.should be_false
-          frame.shard_name.should be_nil
-          frame.in_app?.should be_true
-        end
-      end
-    end
-
-    context "with shard path" do
-      it "parses absolute path inside of lib/ dir" do
-        lib_path = File.expand_path("../../../lib/bar", __DIR__)
-        path = "#{lib_path}/src/bar.cr"
-
-        with_foo_frame(path: path) do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq(path)
-          frame.relative_path.should eq("lib/bar/src/bar.cr")
-          frame.under_src_path?.should be_true
-          frame.shard_name.should eq "bar"
-          frame.in_app?.should be_false
-        end
-      end
-
-      it "parses relative path inside of lib/ dir" do
-        path = "lib/bar/src/bar.cr"
-
-        with_foo_frame(path: path) do |frame|
-          frame.lineno.should eq(1)
-          frame.column.should eq(7)
-          frame.method.should eq("foo_bar?")
-          frame.path.should eq(path)
-          frame.relative_path.should eq(path)
-          frame.under_src_path?.should be_false
-          frame.shard_name.should eq "bar"
-          frame.in_app?.should be_false
-        end
-      end
-
-      it "uses only folders for shard names" do
-        with_foo_frame(path: "lib/bar.cr") do |frame|
-          frame.shard_name.should be_nil
-        end
-      end
-    end
-  end
-
   it "#inspect" do
     with_foo_frame do |frame|
       frame.inspect.should match(/Backtrace::Frame(.*)$/)
@@ -209,7 +8,7 @@ describe Backtracer::Backtrace::Frame do
   end
 
   it "#to_s" do
-    with_foo_frame do |frame|
+    with_foo_frame(path: "#{__DIR__}/foo.cr") do |frame|
       frame.to_s.should eq "`foo_bar?` at #{__DIR__}/foo.cr:1:7"
     end
   end
@@ -219,9 +18,44 @@ describe Backtracer::Backtrace::Frame do
       with_foo_frame do |frame2|
         frame.should eq(frame2)
       end
-      with_foo_frame(method: "other_method") do |frame2|
-        frame.should_not eq(frame2)
+      with_foo_frame(method: "other_method") do |frame3|
+        frame.should_not eq(frame3)
       end
     end
   end
+
+  {% unless flag?(:release) || !flag?(:debug) %}
+    describe "#context" do
+      it "returns proper lines" do
+        with_configuration do |configuration|
+          with_backtrace(caller) do |backtrace|
+            backtrace.frames.first.tap do |first_frame|
+              context_lines = configuration.context_lines.should_not be_nil
+              context = first_frame.context.should_not be_nil
+
+              lines = File.read_lines(__FILE__)
+              lineidx = context.lineno - 1
+
+              context.pre
+                .should eq(lines[Math.max(0, lineidx - context_lines), context_lines]?)
+              context.line
+                .should eq(lines[lineidx]?)
+              context.post
+                .should eq(lines[Math.min(lines.size, lineidx + 1), context_lines]?)
+            end
+          end
+        end
+      end
+
+      it "returns given amount of lines" do
+        with_backtrace(caller) do |backtrace|
+          backtrace.frames.first.tap do |first_frame|
+            context = first_frame.context(3).should_not be_nil
+            context.pre.size.should eq(3)
+            context.post.size.should eq(3)
+          end
+        end
+      end
+    end
+  {% end %}
 end
diff --git a/spec/backtracer/backtrace/parser_spec.cr b/spec/backtracer/backtrace/parser_spec.cr
new file mode 100644
index 0000000..7713983
--- /dev/null
+++ b/spec/backtracer/backtrace/parser_spec.cr
@@ -0,0 +1,21 @@
+require "../../spec_helper"
+
+describe Backtracer::Backtrace::Parser do
+  describe ".parse" do
+    it "handles `caller` as an input" do
+      with_backtrace(caller) do |backtrace|
+        backtrace.frames.should_not be_empty
+      end
+    end
+
+    it "handles `Exception#backtrace` as an input" do
+      begin
+        raise "Oh, no!"
+      rescue ex
+        with_backtrace(ex.backtrace) do |backtrace|
+          backtrace.frames.should_not be_empty
+        end
+      end
+    end
+  end
+end
diff --git a/spec/backtracer/backtrace_spec.cr b/spec/backtracer/backtrace_spec.cr
index 36bc340..c8d58f2 100644
--- a/spec/backtracer/backtrace_spec.cr
+++ b/spec/backtracer/backtrace_spec.cr
@@ -1,24 +1,31 @@
 require "../spec_helper"
 
 describe Backtracer::Backtrace do
-  backtrace = Backtracer.parse(caller)
-
   it "#frames" do
-    backtrace.frames.should be_a(Array(Backtracer::Backtrace::Frame))
+    with_backtrace(caller) do |backtrace|
+      backtrace.frames.should be_a(Array(Backtracer::Backtrace::Frame))
+      backtrace.frames.should_not be_empty
+    end
   end
 
   it "#inspect" do
-    backtrace.inspect.should match(/#<Backtrace: .*>$/)
+    with_backtrace(caller) do |backtrace|
+      backtrace.inspect.should match(/#<Backtrace: .+>$/)
+    end
   end
 
   {% unless flag?(:release) || !flag?(:debug) %}
     it "#to_s" do
-      backtrace.to_s.should match(/backtrace_spec.cr:4/)
+      with_backtrace(caller) do |backtrace|
+        backtrace.to_s.should match(/backtrace_spec.cr/)
+      end
     end
   {% end %}
 
   it "#==" do
-    backtrace2 = Backtracer::Backtrace.new(backtrace.frames)
-    backtrace2.should eq(backtrace)
+    with_backtrace(caller) do |backtrace|
+      backtrace2 = Backtracer::Backtrace.new(backtrace.frames)
+      backtrace2.should eq(backtrace)
+    end
   end
 end
diff --git a/spec/backtracer/configuration_spec.cr b/spec/backtracer/configuration_spec.cr
index 76284c6..c192860 100644
--- a/spec/backtracer/configuration_spec.cr
+++ b/spec/backtracer/configuration_spec.cr
@@ -1,9 +1,5 @@
 require "../spec_helper"
 
-private def with_configuration
-  yield Backtracer::Configuration.new
-end
-
 describe Backtracer::Configuration do
   it "should set #src_path to current dir from default" do
     with_configuration do |configuration|
diff --git a/spec/spec_helper.cr b/spec/spec_helper.cr
index e0c7f38..ed152bb 100644
--- a/spec/spec_helper.cr
+++ b/spec/spec_helper.cr
@@ -1,2 +1,37 @@
 require "spec"
 require "../src/backtracer"
+
+def with_configuration(shared = true, &)
+  yield shared ? Backtracer.configuration : Backtracer::Configuration.new
+end
+
+def with_backtrace(backtrace, **options, &)
+  yield Backtracer::Backtrace::Parser.parse(backtrace, **options)
+end
+
+def with_frame(method, path = nil, lineno = nil, column = nil, **options, &)
+  line = String.build do |io|
+    if path
+      io << path
+      io << ':' << lineno if lineno
+      io << ':' << column if column
+      io << " in '" << method << '\''
+    else
+      io << method
+    end
+  end
+  yield Backtracer::Backtrace::Frame::Parser.parse(line, **options)
+end
+
+def with_foo_frame(
+  method = "foo_bar?",
+  path = "#{__DIR__}/foo.cr",
+  lineno = 1,
+  column = 7,
+  **options,
+  &
+)
+  with_frame(method, path, lineno, column, **options) do |frame|
+    yield frame
+  end
+end
diff --git a/src/backtracer.cr b/src/backtracer.cr
index e11a616..c507a2d 100644
--- a/src/backtracer.cr
+++ b/src/backtracer.cr
@@ -1,6 +1,10 @@
 module Backtracer
   class_getter(configuration) { Configuration.new }
 
+  def self.configure(&) : Nil
+    yield configuration
+  end
+
   def self.parse(backtrace : Array(String) | String, **options) : Backtrace
     Backtrace::Parser.parse(backtrace, **options)
   end
diff --git a/src/backtracer/backtrace/frame.cr b/src/backtracer/backtrace/frame.cr
index c7fd0fd..5826422 100644
--- a/src/backtracer/backtrace/frame.cr
+++ b/src/backtracer/backtrace/frame.cr
@@ -1,6 +1,8 @@
 module Backtracer
   # An object representation of a stack frame.
   struct Backtrace::Frame
+    @context_cache = {} of Int32 => Context
+
     # The method of this frame (such as `User.find`).
     getter method : String
 
@@ -21,7 +23,7 @@ module Backtracer
 
     def_equals_and_hash @method, @path, @lineno, @column
 
-    # Reconstructs the frame in a readable fashion
+    # Reconstructs the frame in a readable fashion.
     def to_s(io : IO) : Nil
       io << '`' << @method << '`'
       if @path
@@ -37,11 +39,24 @@ module Backtracer
       io << ')'
     end
 
+    # Returns `true` if `path` of this frame is within
+    # the `configuration.src_path`, `false` otherwise.
+    #
+    # See `Configuration#src_path`
     def under_src_path? : Bool
       return false unless src_path = configuration.src_path
       !!path.try(&.starts_with?(src_path))
     end
 
+    # Returns:
+    #
+    # - `path` as is, unless it's absolute - i.e. starts with `/`
+    # - `path` relative to `configuration.src_path` when `under_src_path?` is `true`
+    # - `nil` otherwise
+    #
+    # NOTE: returned path is not required to be `under_src_path?` - see point no. 1
+    #
+    # See `Configuration#src_path`
     def relative_path : String?
       return unless path = @path
       return path unless path.starts_with?('/')
@@ -51,6 +66,13 @@ module Backtracer
       end
     end
 
+    # Returns:
+    #
+    # - `path` as is, if it's absolute - i.e. starts with `/`
+    # - `path` appended to `configuration.src_path`
+    # - `nil` otherwise
+    #
+    # See `Configuration#src_path`
     def absolute_path : String?
       return unless path = @path
       return path if path.starts_with?('/')
@@ -59,51 +81,67 @@ module Backtracer
       end
     end
 
+    # Returns name of the shard from which this frame originated.
+    #
+    # See `Configuration#modules_path_pattern`
     def shard_name : String?
       relative_path
         .try(&.match(configuration.modules_path_pattern))
         .try(&.["name"])
     end
 
+    # Returns `true` if this frame originated from the app source code,
+    # `false` otherwise.
+    #
+    # See `Configuration#app_dirs_pattern`
     def in_app? : Bool
-      !!(path.try(&.matches?(configuration.in_app_pattern)))
+      !!(relative_path.try(&.matches?(configuration.app_dirs_pattern)))
     end
 
-    def context(context_lines : Int32? = nil) : {Array(String), String, Array(String)}?
+    # Returns `Context` record consisting of 3 elements - an array of context lines
+    # before the `lineno`, line at `lineno`, and an array of context lines
+    # after the `lineno`. In case of failure it returns `nil`.
+    #
+    # Amount of returned context lines is taken from the *context_lines*
+    # argument if given, or `configuration.context_lines` otherwise.
+    #
+    # NOTE: amount of returned context lines might be lower than given
+    # in cases where `lineno` is near the start or the end of the file.
+    #
+    # See `Configuration#context_lines`
+    def context(context_lines : Int32? = nil) : Context?
       context_lines ||= configuration.context_lines
-
       return unless context_lines && (context_lines > 0)
+
+      cached = @context_cache[context_lines]?
+      return cached if cached
+
       return unless (lineno = @lineno) && (lineno > 0)
       return unless (path = @path) && File.readable?(path)
 
-      lines = File.read_lines(path)
-      lineidx = lineno - 1
+      context_line = nil
+      pre_context, post_context = %w[], %w[]
 
-      if context_line = lines[lineidx]?
-        pre_context = lines[Math.max(0, lineidx - context_lines), context_lines]
-        post_context = lines[Math.min(lines.size, lineidx + 1), context_lines]
-        {pre_context, context_line, post_context}
+      i = 0
+      File.each_line(path) do |line|
+        case i += 1
+        when lineno - context_lines...lineno
+          pre_context << line
+        when lineno
+          context_line = line
+        when lineno + 1..lineno + context_lines
+          post_context << line
+        end
       end
-    end
 
-    def context_hash(context_lines : Int32? = nil) : Hash(Int32, String)?
-      return unless context = self.context(context_lines)
-      return unless lineno = @lineno
-
-      pre_context, context_line, post_context = context
-
-      ({} of Int32 => String).tap do |hash|
-        pre_context.each_with_index do |code, index|
-          line = (lineno - pre_context.size) + index
-          hash[line] = code
-        end
-
-        hash[lineno] = context_line
-
-        post_context.each_with_index do |code, index|
-          line = lineno + (index + 1)
-          hash[line] = code
-        end
+      if context_line
+        @context_cache[context_lines] =
+          Context.new(
+            lineno: lineno,
+            pre: pre_context,
+            line: context_line,
+            post: post_context,
+          )
       end
     end
   end
diff --git a/src/backtracer/backtrace/frame/context.cr b/src/backtracer/backtrace/frame/context.cr
new file mode 100644
index 0000000..d5d22aa
--- /dev/null
+++ b/src/backtracer/backtrace/frame/context.cr
@@ -0,0 +1,46 @@
+module Backtracer
+  struct Backtrace::Frame::Context
+    # The line number this `Context` refers to.
+    getter lineno : Int32
+
+    # An array of lines before `lineno`.
+    getter pre : Array(String)
+
+    # The line at `lineno`.
+    getter line : String
+
+    # An array of lines after `lineno`.
+    getter post : Array(String)
+
+    def initialize(@lineno, @pre, @line, @post)
+    end
+
+    # Returns an array composed of context lines from `pre`,
+    # `line` and `post`.
+    def to_a : Array(String)
+      ([] of String).tap do |ary|
+        ary.concat(pre)
+        ary << line
+        ary.concat(post)
+      end
+    end
+
+    # Returns hash with context lines, where line numbers are
+    # the keys and the lines itself are the values.
+    def to_h : Hash(Int32, String)
+      ({} of Int32 => String).tap do |hash|
+        base_index = lineno - pre.size
+        pre.each_with_index do |code, index|
+          hash[base_index + index] = code
+        end
+
+        hash[lineno] = line
+
+        base_index = lineno + 1
+        post.each_with_index do |code, index|
+          hash[base_index + index] = code
+        end
+      end
+    end
+  end
+end
diff --git a/src/backtracer/backtrace/frame/parser.cr b/src/backtracer/backtrace/frame/parser.cr
index f771639..37f77ff 100644
--- a/src/backtracer/backtrace/frame/parser.cr
+++ b/src/backtracer/backtrace/frame/parser.cr
@@ -2,10 +2,13 @@ module Backtracer
   module Backtrace::Frame::Parser
     extend self
 
-    # Parses a single line of a given backtrace, where *unparsed_line* is
+    # Parses a single line of a given backtrace, where *line* is
     # the raw line from `caller` or some backtrace.
     #
-    # Returns the parsed backtrace frame on success or `nil` otherwise.
+    # Accepts options:
+    # - `configuration`: `Configuration` object - uses `Backtracer.configuration` if `nil`
+    #
+    # Returns parsed `Backtrace::Frame` on success or `nil` otherwise.
     def parse?(line : String, **options) : Backtrace::Frame?
       return unless Configuration::LINE_PATTERNS.any? &.match(line)
 
@@ -20,6 +23,7 @@ module Backtracer
         configuration: options[:configuration]?
     end
 
+    # Same as `parse?` but raises `ArgumentError` on error.
     def parse(line : String, **options) : Backtrace::Frame
       parse?(line, **options) ||
         raise ArgumentError.new("Error parsing line: #{line.inspect}")
diff --git a/src/backtracer/backtrace/parser.cr b/src/backtracer/backtrace/parser.cr
index ade656e..30d6744 100644
--- a/src/backtracer/backtrace/parser.cr
+++ b/src/backtracer/backtrace/parser.cr
@@ -2,6 +2,14 @@ module Backtracer
   module Backtrace::Parser
     extend self
 
+    # Parses *backtrace* (possibly obtained as a return value
+    # from `caller` or `Exception#backtrace` methods).
+    #
+    # Accepts options:
+    # - `configuration`: `Configuration` object - uses `Backtracer.configuration` if `nil`
+    # - `filters`: additional line filters - see `Configuration#line_filters`
+    #
+    # Returns parsed `Backtrace` object or raises `ArgumentError` otherwise.
     def parse(backtrace : Array(String), **options) : Backtrace
       configuration = options[:configuration]? || Backtracer.configuration
 
diff --git a/src/backtracer/configuration.cr b/src/backtracer/configuration.cr
index c944188..1b91465 100644
--- a/src/backtracer/configuration.cr
+++ b/src/backtracer/configuration.cr
@@ -53,24 +53,32 @@ module Backtracer
       /^(?<method>.+?)$/,
     }
 
-    # Used in `#in_app_pattern`.
+    # Path considered as "root" of your project.
+    #
+    # See `Frame#under_src_path?`
     property src_path : String? = {{ Process::INITIAL_PWD }}
 
     # Directories to be recognized as part of your app. e.g. if you
     # have an `engines` dir at the root of your project, you may want
-    # to set this to something like `/(src|engines)/`
-    property app_dirs_pattern = /src/
-
-    # `Regex` pattern matched against `Backtrace::Frame#file`.
-    property in_app_pattern : Regex { /^(#{src_path}\/)?(#{app_dirs_pattern})/ }
+    # to set this to something like `/^(src|engines)\//`
+    #
+    # See `Frame#in_app?`
+    property app_dirs_pattern = /^src\//
 
     # Path pattern matching directories to be recognized as your app modules.
     # Defaults to standard Shards setup (`lib/shard-name/...`).
+    #
+    # See `Frame#shard_name`
     property modules_path_pattern = /^lib\/(?<name>[^\/]+)\/(?:.+)/
 
-    # Number of lines of code context to capture, or `nil` for none.
+    # Number of lines of code context to return by default, or `nil` for none.
+    #
+    # See `Frame#context`
     property context_lines : Int32? = 5
 
+    # Array of procs used for filtering backtrace lines before parsing.
+    # Each filter is expected to return a string, which is then passed
+    # onto the next filter, or ignored althoghether if `nil` is returned.
     getter(line_filters) {
       [
         ->(line : String) { line unless line.matches?(IGNORED_LINES_PATTERN) },