From 770100891c588c52f52a0b43c5ac626d417241d5 Mon Sep 17 00:00:00 2001
From: Michael Miller <icy.arctic.fox@gmail.com>
Date: Sun, 14 Jul 2019 12:24:43 -0600
Subject: [PATCH] Add change.from variant

---
 src/spectator/matchers/change_from_matcher.cr | 97 +++++++++++++++++++
 src/spectator/matchers/change_matcher.cr      |  6 ++
 2 files changed, 103 insertions(+)
 create mode 100644 src/spectator/matchers/change_from_matcher.cr

diff --git a/src/spectator/matchers/change_from_matcher.cr b/src/spectator/matchers/change_from_matcher.cr
new file mode 100644
index 0000000..858fa6a
--- /dev/null
+++ b/src/spectator/matchers/change_from_matcher.cr
@@ -0,0 +1,97 @@
+require "./value_matcher"
+
+module Spectator::Matchers
+  # Matcher that tests whether an expression changed from a specific value.
+  struct ChangeFromMatcher(ExpressionType, FromType) < ValueMatcher(ExpressionType)
+    # Determines whether the matcher is satisfied with the partial given to it.
+    # `MatchData` is returned that contains information about the match.
+    def match(partial)
+      partial.actual           # Invoke action that might change the expression's value.
+      after = @expression.call # Retrieve the expression's value.
+      if expected != @actual_before
+        # Initial value isn't what was expected.
+        InitialMatchData.new(expected, @actual_before, after, partial.label, label)
+      else
+        # Check if the expression's value changed.
+        matched = expected != after
+        ChangeMatchData.new(matched, expected, @actual_before, after, partial.label, label)
+      end
+    end
+
+    # Creates a new change matcher with a custom label.
+    def initialize(expression_label : String, expected_before : FromType, @actual_before : ExpressionType, &expression : -> ExpressionType)
+      super(expected_before, expression_label)
+      @expression = expression
+    end
+
+    # Creates a new change matcher.
+    def initialize(expected_before : FromType, @actual_before : ExpressionType, &expression : -> ExpressionType)
+      super(expected_before, expression.to_s)
+      @expression = expression
+    end
+
+    # Match data for when the initial value isn't the expected value.
+    private struct InitialMatchData(ExpressionType, FromType) < MatchData
+      # Creates the match data.
+      def initialize(@expected_before : FromType, @actual_before : ExpressionType, @after : ExpressionType,
+                     @action_label : String, @expression_label : String)
+        super(false)
+      end
+
+      # Do not allow negation of this match data.
+      def override?
+        true
+      end
+
+      # Information about the match.
+      def named_tuple
+        {
+          "expected before": @expected_before,
+          "actual before":   @actual_before,
+          "expected after":  NegatableMatchDataValue.new(@expected_before, true),
+          "actual after":    @after,
+        }
+      end
+
+      # This is informational and displayed to the end-user.
+      def message
+        "#{@expression_label} is initially #{@expected_before}"
+      end
+
+      # This is informational and displayed to the end-user.
+      def negated_message
+        "#{@expression_label} is not initially #{@expected_before}"
+      end
+    end
+
+    private struct ChangeMatchData(ExpressionType, FromType) < MatchData
+      # Creates the match data.
+      def initialize(matched, @expected_before : FromType, @actual_before : ExpressionType,
+                     @after : ExpressionType, @action_label : String, @expression_label : String)
+        super(matched)
+      end
+
+      # Information about the match.
+      def named_tuple
+        {
+          "expected before": @expected_before,
+          "actual before":   @actual_before,
+          "expected after":  NegatableMatchDataValue.new(@expected_before, true),
+          "actual after":    @after,
+        }
+      end
+
+      # Describes the condition that satisfies the matcher.
+      # This is informational and displayed to the end-user.
+      def message
+        "#{@action_label} changed #{@expression_label} from #{@expected_before}"
+      end
+
+      # Describes the condition that won't satsify the matcher.
+      # This is informational and displayed to the end-user.
+      def negated_message
+        "#{@action_label} did not change #{@expression_label} from #{@expected_before}"
+      end
+    end
+  end
+end
diff --git a/src/spectator/matchers/change_matcher.cr b/src/spectator/matchers/change_matcher.cr
index f8bcb8f..64dda8e 100644
--- a/src/spectator/matchers/change_matcher.cr
+++ b/src/spectator/matchers/change_matcher.cr
@@ -1,3 +1,4 @@
+require "./change_from_matcher"
 require "./value_matcher"
 
 module Spectator::Matchers
@@ -28,6 +29,11 @@ module Spectator::Matchers
       @expression = expression
     end
 
+    # Specifies what the initial value of the expression must be.
+    def from(value : T) forall T
+      ChangeFromMatcher.new(label, value, expected, &@expression)
+    end
+
     # Match data specific to this matcher.
     private struct MatchData(ExpressionType) < MatchData
       # Creates the match data.