diff --git a/src/common/workertypes.ts b/src/common/workertypes.ts
index 63e6f532..eca034a9 100644
--- a/src/common/workertypes.ts
+++ b/src/common/workertypes.ts
@@ -33,6 +33,7 @@ export class SourceFile {
   text: string;
   offset2loc: Map<number,SourceLine>; //{[offset:number]:number};
   line2offset: Map<number,number>; //{[line:number]:number};
+  sortedOffsets: number[];
   
   constructor(lines:SourceLine[], text:string) {
     lines = lines || [];
@@ -44,25 +45,33 @@ export class SourceFile {
       if (info.offset >= 0) {
         // first line wins (is assigned to offset)
         // TODO: handle macros/includes w/ multiple offsets per line
-        if (!this.offset2loc[info.offset])
-          this.offset2loc[info.offset] = info;
-        if (!this.line2offset[info.line])
-          this.line2offset[info.line] = info.offset;
+        if (!this.offset2loc.has(info.offset))
+          this.offset2loc.set(info.offset, info);
+        if (!this.line2offset.has(info.line))
+          this.line2offset.set(info.line, info.offset);
       }
     }
+    this.sortedOffsets = Array.from(this.offset2loc.keys()).sort((a,b)=>a-b);
   }
-  // TODO: smarter about looking for source lines between two addresses
+  // returns the line whose offset is nearest to (but not greater than) PC,
+  // provided it is within `lookbehind` bytes; null otherwise
   findLineForOffset(PC:number, lookbehind:number) : SourceLine {
-    if (this.offset2loc) {
-      for (var i=0; i<=lookbehind; i++) {
-        var loc = this.offset2loc[PC];
-        if (loc) {
-          return loc;
-        }
-        PC--;
+    const offsets = this.sortedOffsets;
+    // binary search for last offset <= PC
+    var lo = 0, hi = offsets.length-1, ans = -1;
+    while (lo <= hi) {
+      var mid = (lo + hi) >> 1;
+      if (offsets[mid] <= PC) {
+        ans = mid;
+        lo = mid + 1;
+      } else {
+        hi = mid - 1;
       }
     }
-    return null;
+    if (ans < 0) return null;
+    var off = offsets[ans];
+    if (PC - off > lookbehind) return null;
+    return this.offset2loc.get(off);
   }
   lineCount():number { return this.lines.length; }
 }
diff --git a/src/ide/breakpoints.ts b/src/ide/breakpoints.ts
index 29845ac2..7011d0a7 100644
--- a/src/ide/breakpoints.ts
+++ b/src/ide/breakpoints.ts
@@ -175,7 +175,7 @@ export function resolveBreakpoint(bp: Breakpoint): ResolvedBreakpoint {
             let lst = current_project.getListingForFile(bp.file);
             let sf = lst && (lst.sourcefile || lst.assemblyfile);
             if (!sf) return { bp, error: "no debug info (build first?)" };
-            pc = sf.line2offset[bp.line];
+            pc = sf.line2offset.get(bp.line);
             if (!(pc >= 0)) return { bp, error: "line has no code" };
         } else {
             let r = parseTarget(bp.target, makeSymbolLookup());
diff --git a/src/ide/views/editors.ts b/src/ide/views/editors.ts
index 670e862f..31f070ea 100644
--- a/src/ide/views/editors.ts
+++ b/src/ide/views/editors.ts
@@ -294,7 +294,7 @@ export class SourceEditor implements ProjectView {
             if (effect.is(currentPcMarker.runToLine)) {
               const lineNum = effect.value;
               if (this.sourcefile && this.sourcefile.line2offset) {
-                const pc = this.sourcefile.line2offset[lineNum];
+                const pc = this.sourcefile.line2offset.get(lineNum);
                 if (pc >= 0) {
                   runToPC([pc]);
                 }
@@ -576,8 +576,7 @@ export class SourceEditor implements ProjectView {
   setTimingResult(result: CodeAnalyzer): void {
     if (this.sourcefile == null) return;
     var newBytes = new Map<number, string>();
-    for (const line of Object.keys(this.sourcefile.line2offset)) {
-      let pc = this.sourcefile.line2offset[line];
+    for (const [line, pc] of this.sourcefile.line2offset) {
       let clocks = result.pc2clockrange[pc];
       var minclocks = clocks && clocks.minclocks;
       var maxclocks = clocks && clocks.maxclocks;
@@ -589,7 +588,7 @@ export class SourceEditor implements ProjectView {
           s = minclocks + "-" + maxclocks;
         if (maxclocks == result.MAX_CLOCKS)
           s += "+";
-        newBytes.set(parseInt(line), s);
+        newBytes.set(line, s);
       }
     }
     this.editor.dispatch({
@@ -719,7 +718,7 @@ export class SourceEditor implements ProjectView {
   getCursorPC(): number {
     var line = this.getCurrentLine();
     while (this.sourcefile && line >= 0) {
-      var pc = this.sourcefile.line2offset[line];
+      var pc = this.sourcefile.line2offset.get(line);
       if (pc >= 0) return pc;
       line--;
     }
diff --git a/test/unit/testcommon_ext.ts b/test/unit/testcommon_ext.ts
index ec3186e8..77af0259 100644
--- a/test/unit/testcommon_ext.ts
+++ b/test/unit/testcommon_ext.ts
@@ -113,9 +113,9 @@ describe('SourceFile Class', function () {
     ];
     const sf = new SourceFile(lines, '');
 
-    assert.ok(sf.offset2loc[0]);
-    assert.ok(sf.offset2loc[2]);
-    assert.ok(sf.offset2loc[4]);
+    assert.ok(sf.offset2loc.get(0));
+    assert.ok(sf.offset2loc.get(2));
+    assert.ok(sf.offset2loc.get(4));
   });
 
   it('should build line to offset mapping', function () {
@@ -126,9 +126,9 @@ describe('SourceFile Class', function () {
     ];
     const sf = new SourceFile(lines, '');
 
-    assert.strictEqual(sf.line2offset[1], 0);
-    assert.strictEqual(sf.line2offset[2], 2);
-    assert.strictEqual(sf.line2offset[3], 4);
+    assert.strictEqual(sf.line2offset.get(1), 0);
+    assert.strictEqual(sf.line2offset.get(2), 2);
+    assert.strictEqual(sf.line2offset.get(3), 4);
   });
 
   it('should handle negative offsets gracefully', function () {
@@ -140,8 +140,8 @@ describe('SourceFile Class', function () {
     const sf = new SourceFile(lines, '');
 
     // Negative offsets should not be added to mapping
-    assert.ok(!sf.offset2loc[-1]);
-    assert.ok(sf.offset2loc[0]);
+    assert.ok(!sf.offset2loc.has(-1));
+    assert.ok(sf.offset2loc.get(0));
   });
 
   it('findLineForOffset should locate source line by address', function () {