diff --git a/core/src/magic.sld b/core/src/magic.sld
index a16c068..7c2b4b7 100644
--- a/core/src/magic.sld
+++ b/core/src/magic.sld
@@ -278,43 +278,26 @@
      is-ca)
 
    (define (rewrite-string str with-rewrites)
-     (define parts '())
-     (define (find-part-at i last-i)
-       (define next-slash (string-contains str "/" i))
-       (if (or (not next-slash) (>= next-slash (- (string-length str) 53)))
-         (if (= last-i 0)
-           (set! parts #f)
-           (set! parts (cons (string-copy str last-i) parts)))
-         (let* ((actual-string (string-copy str next-slash (+ next-slash 53)))
-                (mapping-pair (assoc actual-string with-rewrites string=?)))
-           ; If we have a mapping for this string, replace it and continue.
-           (if mapping-pair
-             (begin
-               (set! parts (cons (cdr mapping-pair) (cons (string-copy str last-i next-slash) parts)))
-               (find-part-at (+ next-slash 53) (+ next-slash 53)))
-             (find-part-at (+ next-slash 1) last-i)))))
-     (find-part-at 0 0)
-     (if (pair? parts)
-       (string-concatenate-reverse parts)
-       str))
+    (define bv (string->utf8 str))
+    (define out (rewrite-bytevector bv with-rewrites))
+    (if (eq? bv out) str (utf8->string out)))
 
    (define (rewrite-bytevector str with-rewrites)
      (define parts '())
-     (define (find-part-at i last-i)
-       (define next-slash (bytestring-index str (lambda (c) (= c #x2F)) i))
+     (let find-part-at ((i 0) (last-i 0))
+      (let ((next-slash (bytestring-index str (lambda (c) (= c #x2F)) i)))
        (if (or (not next-slash) (>= next-slash (- (bytevector-length str) 53)))
          (if (= last-i 0)
            (set! parts #f)
            (set! parts (cons (bytevector-copy str last-i) parts)))
          (let* ((actual-string (utf8->string (bytevector-copy str next-slash (+ next-slash 53))))
-                (mapping-pair (assoc actual-string with-rewrites string=?)))
+                (mapping-pair (mapping-ref/default with-rewrites actual-string #f)))
            ; If we have a mapping for this string, replace it and continue.
            (if mapping-pair
              (begin
-               (set! parts (cons (string->utf8 (cdr mapping-pair)) (cons (bytevector-copy str last-i next-slash) parts)))
+               (set! parts (cons (string->utf8 mapping-pair) (cons (bytevector-copy str last-i next-slash) parts)))
                (find-part-at (+ next-slash 53) (+ next-slash 53)))
-             (find-part-at (+ next-slash 1) last-i)))))
-     (find-part-at 0 0)
+             (find-part-at (+ next-slash 1) last-i))))))
      (if (pair? parts)
        (apply bytevector-append (reverse parts))
        str))
@@ -416,7 +399,7 @@
          ; Rewrite CA drv to IA drv using the known inputs
          (define new-drvs (list))
          (define new-srcs (derivation-input-src (pending-item-ca-drv item)))
-         (define rewrites (list))
+         (define rewrites (mapping (make-default-comparator)))
          (for-each
           (lambda (drv-and-outputs)
             (mutex-lock! pending-mutex)
@@ -429,7 +412,7 @@
                 (define new-path (cdr (assoc output (pending-item-resolved-paths dep-pend))))
                 (set! new-srcs (cons new-path new-srcs))
                 (define old-output (cdr (assoc output (derivation-outputs (car drv-and-outputs)))))
-                (set! rewrites (cons (cons (derivation-output-path old-output) new-path) rewrites)))
+                (set! rewrites (mapping-set! rewrites (derivation-output-path old-output) new-path)))
                (cdr drv-and-outputs))
 
               ; Not a CA drv, add it back to the drvs list