diff --git a/20-trench-map/code.scm b/20-trench-map/code.scm
new file mode 100644
index 0000000..6d21a53
--- /dev/null
+++ b/20-trench-map/code.scm
@@ -0,0 +1,76 @@
+(load "../common.scm")
+
+; returns a vector of the same length of input string
+; where all values are 0 except for when the corresponding
+; entry in the string is '#' which will have a 1 instead.
+(define (string->bin-vector str)
+  (vector-unfold
+    (lambda (i)
+      (if (char=? (string-ref str i) #\#) 1 0))
+    (string-length str)))
+
+; returns a binary matrix loaded from the file
+; all the lines are expected to be the same lengths
+(define (file->matrix file)
+  (let loop [(data '())]
+    (if (eof-object? (peek-char file))
+      (matrix-from-data (reverse-list->vector data))
+      (loop (cons (string->bin-vector (get-line file)) data)))))
+
+; palette enhancing algorihtm: 
+; for each pixel in input image
+; gather all pixels around it to form a 3x3 matrix
+; turn that matrix into a binary number, row by row
+; look up new pixel value in input palette
+(define (enhance image palette steps)
+  (let* [(new-width (+ (matrix-width image) 4))
+	 (new-height (+ (matrix-height image) 4))
+	 (new-image (filled-matrix new-width new-height 0))
+	 (top-left (matrix-get image 0 0 0))]
+    (update-matrix!
+      new-image
+      (lambda (x y v)
+	; not really convinced that defaulting to the top left pixel is the right way to do it
+	(vector-ref palette (+ (* (matrix-get image (- x 3) (- y 3) top-left) 256)
+			       (* (matrix-get image (- x 2) (- y 3) top-left) 128)
+			       (* (matrix-get image (- x 1) (- y 3) top-left) 64)
+			       (* (matrix-get image (- x 3) (- y 2) top-left) 32)
+			       (* (matrix-get image (- x 2) (- y 2) top-left) 16)
+			       (* (matrix-get image (- x 1) (- y 2) top-left) 8)
+			       (* (matrix-get image (- x 3) (- y 1) top-left) 4)
+			       (* (matrix-get image (- x 2) (- y 1) top-left) 2)
+			       (* (matrix-get image (- x 1) (- y 1) top-left) 1)))))
+    (if (<= steps 1)
+      new-image
+      (enhance new-image palette (- steps 1)))))
+
+; for debugging purposes
+; output a PGM 1bpp image file from a given binary matrix
+(define (matrix->image-file matrix filename)
+  (when (file-exists? filename)
+    (delete-file filename))
+  (call-with-output-file
+    filename
+    (lambda (file)
+      (fprintf file "P1~%~a ~a~%" (matrix-width matrix) (matrix-height matrix))
+      (vector-for-each
+	(lambda (y line)
+	  (vector-for-each 
+	    (lambda (x pixel) (fprintf file "~a " (- 1 pixel))) line)
+	  (fprintf file "~%"))
+	(matrix-data matrix)))))
+
+(call-with-input-file
+  "input"
+  (lambda (file)
+    (let* [(enhancer (string->bin-vector (get-line file)))
+	  (image (begin
+		   (get-line file) ; skip empty line
+		   (file->matrix file)))]
+      (let [(enhanced (enhance image enhancer 2))]
+	(printf "part 1:~% Lit pixels after enhancing twice: ~a~%"
+		(matrix-sum enhanced)))
+      (let [(enhanced (enhance image enhancer 50))]
+	(printf "part 2:~% Lit pixels after enhancing fifty times: ~a~%"
+		(matrix-sum enhanced))))))
+
diff --git a/20-trench-map/input b/20-trench-map/input
new file mode 100644
index 0000000..076365a
--- /dev/null
+++ b/20-trench-map/input
@@ -0,0 +1,102 @@
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