add basic plugin position code

src/image.zig

@@ -4,6 +4,9 @@ const c = lv2.c;
 
 const plugins = @import("plugin.zig");
 
+/// Approximate size of the BMP header.
+pub const BMPHeaderSize: usize = 82000;
+
 pub const ImageError = error{
     OpenFail,
     InvalidPlugin,
@@ -38,6 +41,12 @@ fn sopen(
     return file.?;
 }
 
+fn getEndPos(path: []const u8) !usize {
+    var file = try std.fs.File.openRead(path);
+    defer file.close();
+    return file.getEndPos();
+}
+
 fn temporaryName(allocator: *std.mem.Allocator) ![]u8 {
     const template_start = "/temp/temp_";
     const template = "/tmp/temp_XXXXXX";
@@ -135,7 +144,7 @@ pub const Image = struct {
     pub fn runPlugin(
         self: *Image,
         plugin_uri: []const u8,
-        pos: plugins.Position,
+        position: plugins.Position,
         params: plugins.ParamList,
     ) !void {
         var ctx = try plugins.makeContext(self.allocator, plugin_uri);
@@ -186,5 +195,14 @@ pub const Image = struct {
         var rctx = try plugins.RunContext.init(self.allocator, ctx.plugin);
         rctx.connectPorts(ports);
         lv2.lilv_instance_activate(rctx.instance);
+
+        // now that we have everything setup, we need to make the part where we
+        // just copy the original image and the part where we run the plugin
+        // over the image.
+
+        const end_pos = (try getEndPos(self.path)) - BMPHeaderSize;
+        std.debug.warn("file end: {}\n", end_pos);
+        const seek_pos = position.seekPos(end_pos);
+        std.debug.warn("start {} end {}\n", seek_pos.start, seek_pos.end);
     }
 };

src/plugin.zig

@@ -17,10 +17,25 @@ pub const Param = struct {
 /// List of parameters to be set to control ports.
 pub const ParamList = std.ArrayList(Param);
 
+/// Represents an absolute position in the image.
+pub const SeekPos = struct {
+    start: usize,
+    end: usize,
+};
+
 /// Represents a relative position in the image
 pub const Position = struct {
     split: usize,
     index: usize,
+
+    pub fn seekPos(self: Position, total_size: usize) SeekPos {
+        var tot = total_size / self.split;
+
+        return SeekPos{
+            .start = self.index * tot,
+            .end = (self.index + 1) * tot,
+        };
+    }
 };
 
 /// Represents the starting context for a single plugin run.