Subject: [PATCH 2/4] guix: build: Avoid using magic literals in the code for hash length.

From: Attila Lendvai <[email-masked]>
Date: Sun, 3 Oct 2021 14:43:01 +0200

* guix/build/utils.scm (%store-hash-string-length): New constant.
(store-path-prefix-length): Factor out the calculation of the total store
prefix length.
* guix/build/graft.scm (hash-length): Use it.
---
 guix/build/graft.scm |  2 +-
 guix/build/utils.scm | 11 +++++++++--
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/guix/build/graft.scm b/guix/build/graft.scm
index daac958d4f..281dbaba6f 100644
--- a/guix/build/graft.scm
+++ b/guix/build/graft.scm
@@ -44,7 +44,7 @@
 ;;;
 ;;; Code:
 
-(define-constant hash-length 32)
+(define-constant hash-length %store-hash-string-length)
 
 (define nix-base32-char?
   (cute char-set-contains?
diff --git a/guix/build/utils.scm b/guix/build/utils.scm
index f3d913aa70..4009c137b8 100644
--- a/guix/build/utils.scm
+++ b/guix/build/utils.scm
@@ -44,6 +44,7 @@
                ;; <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=26805#16>.
                delete)
   #:export (%store-directory
+            %store-hash-string-length
             store-file-name?
             strip-store-file-name
             package-name->name+version
@@ -154,15 +155,21 @@
   (or (getenv "NIX_STORE")
       "/gnu/store"))
 
+(define-constant %store-hash-string-length 32)
+
 (define (store-file-name? file)
   "Return true if FILE is in the store."
   (string-prefix? (%store-directory) file))
 
+(define (store-path-prefix-length)
+  (+ 2         ; the slash after %store-directory, and the dash after the hash
+     (string-length (%store-directory))
+     %store-hash-string-length))
+
 (define (strip-store-file-name file)
   "Strip the '/gnu/store' and hash from FILE, a store file name.  The result
 is typically a \"PACKAGE-VERSION\" string."
-  (string-drop file
-               (+ 34 (string-length (%store-directory)))))
+  (string-drop file (store-path-prefix-length)))
 
 (define (package-name->name+version name)
   "Given NAME, a package name like \"foo-0.9.1b\", return two values:
-- 
2.33.0