Calculate the actual frame line.

Author: smashwilson

lib/stacktrace.coffee

@@ -95,9 +95,9 @@ class Frame
   # Public: Asynchronously collect n lines of context around the specified line number in this
   # frame's source file.
   #
-  # n        - The number of lines of context to collect on *each* side of the error line. The error
-  #            line will always be `lines[n]` and `lines.length` will be `n * 2 + 1`.
-  # callback - Invoked with any errors or an Array containing the relevant lines.
+  # n        - The number of lines of context to collect on *each* side of the error line.
+  # callback - Invoked with any errors, or an Array containing the relevant lines and the index of
+  #            the line in the Array that corresponds to the actual frame.
   #
   getContext: (n, callback) ->
     # Notice that @lineNumber is one-indexed, not zero-indexed.
@@ -106,7 +106,18 @@ class Frame
       toLine: @lineNumber + n
       trim: false
       keepLastEmptyLine: true
-    chomp fs.createReadStream(@realPath), range, callback
+    chomp fs.createReadStream(@realPath), range, (err, lines) =>
+      if err?
+        callback(err)
+      else
+        # Determine which line is the actual trace line.
+        if lines.length < (2 * n + 1) and range.fromLine < 0
+          # The front is cut off.
+          traceLine = @lineNumber - 1
+        else
+          traceLine = n
+
+        callback(null, lines, traceLine)
 
   navigateTo: ->
     position = [@lineNumber - 1, 0]

spec/stacktrace-spec.coffee

@@ -103,18 +103,19 @@ describe 'Stacktrace', ->
         expect(event.newTrace).toBe(trace)
 
 describe 'Frame', ->
-  [frame] = []
+  [fixturePath] = []
 
   beforeEach ->
     fixturePath = path.join __dirname, 'fixtures', 'context.txt'
-    frame = new Frame('five', fixturePath, 5, 'something')
 
   it 'acquires n lines of context asynchronously', ->
-    lines = null
+    [lines, traceLine] = []
+    frame = new Frame('five', fixturePath, 5, 'something')
 
-    frame.getContext 2, (err, ls) ->
+    frame.getContext 2, (err, ls, lnum) ->
       throw err if err?
       lines = ls
+      traceLine = lnum
 
     waitsFor -> lines?
 
@@ -125,3 +126,40 @@ describe 'Frame', ->
       expect(lines[2]).toEqual('five')
       expect(lines[3]).toEqual('six')
       expect(lines[4]).toEqual('')
+      expect(traceLine).toBe(2)
+
+  it 'identifies the trace line if the beginning is cut off', ->
+    [lines, traceLine] = []
+    frame = new Frame('two', fixturePath, 2, 'something')
+
+    frame.getContext 3, (err, ls, lnum) ->
+      throw err if err?
+      lines = ls
+      traceLine = lnum
+
+    waitsFor -> lines?
+
+    runs ->
+      expect(lines.length).toBe(5)
+      expect(lines[0]).toEqual('one')
+      expect(lines[4]).toEqual('five')
+      expect(traceLine).toBe(1)
+      expect(lines[1]).toEqual('two')
+
+  it 'identifies the trace line if the end is cut off', ->
+    [lines, traceLine] = []
+    frame = new Frame('nine', fixturePath, 9, 'something')
+
+    frame.getContext 3, (err, ls, lnum) ->
+      throw err if err?
+      lines = ls
+      traceLine = lnum
+
+    waitsFor -> lines?
+
+    runs ->
+      expect(lines.length).toBe(6)
+      expect(lines[0]).toEqual('six')
+      expect(lines[5]).toEqual('')
+      expect(traceLine).toBe(3)
+      expect(lines[3]).toEqual('nine')