diff --git a/lib/less/source-map-output.js b/lib/less/source-map-output.js
index 4e21e24c..e0bdd362 100644
--- a/lib/less/source-map-output.js
+++ b/lib/less/source-map-output.js
@@ -1,10 +1,38 @@
 module.exports = function (environment) {
 
+    /**
+     * @param source The code
+     * @param ignoredCharsCount Number of characters at the start of the file to ignore.
+     * @constructor
+     */
+    var FileInfo = function (source, ignoredCharsCount) {
+        this.ignoredCharsCount = ignoredCharsCount;
+        this.source = source.slice(ignoredCharsCount);
+        this.sourceLines = this.source.split('\n');
+    };
+
+    /** Translate absolute source offset to line/column offset. */
+    FileInfo.prototype.getLocation = function (index) {
+        index = Math.max(0, index - this.ignoredCharsCount);
+        var line = 0;
+        for (; line < this.sourceLines.length && index >= this.sourceLines[line].length + 1; line++) {
+            index -= this.sourceLines[line].length + 1; // +1 for the '\n' character
+        }
+        return {line: line + 1, column: index};
+    };
+
     var SourceMapOutput = function (options) {
         this._css = [];
         this._rootNode = options.rootNode;
-        this._contentsMap = options.contentsMap;
-        this._contentsIgnoredCharsMap = options.contentsIgnoredCharsMap;
+
+        this._contentsInfoMap = {};
+        for (var key in options.contentsMap) {
+            if (options.contentsMap.hasOwnProperty(key)) {
+                this._contentsInfoMap[key] = new FileInfo(
+                    options.contentsMap[key], options.contentsIgnoredCharsMap[key] || 0);
+            }
+        }
+
         if (options.sourceMapFilename) {
             this._sourceMapFilename = options.sourceMapFilename.replace(/\\/g, '/');
         }
@@ -48,39 +76,22 @@ module.exports = function (environment) {
         }
 
         var lines,
-            sourceLines,
             columns,
-            sourceColumns,
             i;
 
-        if (fileInfo) {
-            var inputSource = this._contentsMap[fileInfo.filename];
-
-            // remove vars/banner added to the top of the file
-            if (this._contentsIgnoredCharsMap[fileInfo.filename]) {
-                // adjust the index
-                index -= this._contentsIgnoredCharsMap[fileInfo.filename];
-                if (index < 0) { index = 0; }
-                // adjust the source
-                inputSource = inputSource.slice(this._contentsIgnoredCharsMap[fileInfo.filename]);
-            }
-            inputSource = inputSource.substring(0, index);
-            sourceLines = inputSource.split("\n");
-            sourceColumns = sourceLines[sourceLines.length - 1];
-        }
-
         lines = chunk.split("\n");
         columns = lines[lines.length - 1];
 
         if (fileInfo) {
+            var location = this._contentsInfoMap[fileInfo.filename].getLocation(index);
             if (!mapLines) {
                 this._sourceMapGenerator.addMapping({ generated: { line: this._lineNumber + 1, column: this._column},
-                    original: { line: sourceLines.length, column: sourceColumns.length},
+                    original: location,
                     source: this.normalizeFilename(fileInfo.filename)});
             } else {
                 for (i = 0; i < lines.length; i++) {
                     this._sourceMapGenerator.addMapping({ generated: { line: this._lineNumber + i + 1, column: i === 0 ? this._column : 0},
-                        original: { line: sourceLines.length + i, column: i === 0 ? sourceColumns.length : 0},
+                        original: { line: location.line + i, column: i === 0 ? location.column : 0},
                         source: this.normalizeFilename(fileInfo.filename)});
                 }
             }
@@ -105,11 +116,8 @@ module.exports = function (environment) {
 
         if (this._outputSourceFiles) {
             for (var filename in this._contentsMap) {
-                if (this._contentsMap.hasOwnProperty(filename)) {
-                    var source = this._contentsMap[filename];
-                    if (this._contentsIgnoredCharsMap[filename]) {
-                        source = source.slice(this._contentsIgnoredCharsMap[filename]);
-                    }
+                if (this._contentsInfoMap.hasOwnProperty(filename)) {
+                    var source = this._contentsInfoMap[filename].source;
                     this._sourceMapGenerator.setSourceContent(this.normalizeFilename(filename), source);
                 }
             }