diff --git a/src/BitSet.js b/src/BitSet.js
new file mode 100644
index 0000000..eccd909
--- /dev/null
+++ b/src/BitSet.js
@@ -0,0 +1,15 @@
+export default class BitSet {
+	constructor(arg) {
+		this.bits = arg instanceof BitSet ? arg.bits.slice() : [];
+	}
+
+	add(n) {
+		this.bits[Math.floor(n / BITS)] |= 1 << n % BITS;
+	}
+
+	has(n) {
+		return !!(this.bits[Math.floor(n / BITS)] & (1 << n % BITS));
+	}
+}
+
+const BITS = 32;
diff --git a/src/MagicString.js b/src/MagicString.js
index 18bf1f4..14f5cb2 100644
--- a/src/MagicString.js
+++ b/src/MagicString.js
@@ -1,3 +1,4 @@
+import BitSet from './BitSet.js';
 import Chunk from './Chunk.js';
 import SourceMap from './SourceMap.js';
 import guessIndent from './utils/guessIndent.js';
@@ -30,7 +31,7 @@ export default class MagicString {
 			byEnd:                 { writable: true, value: {} },
 			filename:              { writable: true, value: options.filename },
 			indentExclusionRanges: { writable: true, value: options.indentExclusionRanges },
-			sourcemapLocations:    { writable: true, value: {} },
+			sourcemapLocations:    { writable: true, value: new BitSet() },
 			storedNames:           { writable: true, value: {} },
 			indentStr:             { writable: true, value: guessIndent(string) }
 		});
@@ -44,7 +45,7 @@ export default class MagicString {
 	}
 
 	addSourcemapLocation(char) {
-		this.sourcemapLocations[char] = true;
+		this.sourcemapLocations.add(char);
 	}
 
 	append(content) {
@@ -121,9 +122,7 @@ export default class MagicString {
 			cloned.indentExclusionRanges = this.indentExclusionRanges.slice();
 		}
 
-		Object.keys(this.sourcemapLocations).forEach(loc => {
-			cloned.sourcemapLocations[loc] = true;
-		});
+		cloned.sourcemapLocations = new BitSet(this.sourcemapLocations);
 
 		cloned.intro = this.intro;
 		cloned.outro = this.outro;
diff --git a/src/utils/Mappings.js b/src/utils/Mappings.js
index 6026200..7de80d9 100644
--- a/src/utils/Mappings.js
+++ b/src/utils/Mappings.js
@@ -28,7 +28,7 @@ export default class Mappings {
 		let first = true;
 
 		while (originalCharIndex < chunk.end) {
-			if (this.hires || first || sourcemapLocations[originalCharIndex]) {
+			if (this.hires || first || sourcemapLocations.has(originalCharIndex)) {
 				this.rawSegments.push([this.generatedCodeColumn, sourceIndex, loc.line, loc.column]);
 			}