store: Add `store-path-hash-part'.

* guix/store.scm (store-path-hash-part): New procedure.
* tests/store.scm ("store-path-hash-part", "store-path-hash-part #f"):
  New tests.

1 files changed, 11 insertions, 1 deletions

diff --git a/guix/store.scm b/guix/store.scm
index 4d078c5899..3bb2656bb6 100644
--- a/guix/store.scm
+++ b/guix/store.scm
@@ -83,7 +83,8 @@
             %store-prefix
             store-path?
             derivation-path?
-            store-path-package-name))
+            store-path-package-name
+            store-path-hash-part))
 
 (define %protocol-version #x10c)
 
@@ -751,3 +752,12 @@ collected, and the number of bytes freed."
 
   (and=> (regexp-exec store-path-rx path)
          (cut match:substring <> 1)))
+
+(define (store-path-hash-part path)
+  "Return the hash part of PATH as a base32 string, or #f if PATH is not a
+syntactically valid store path."
+  (let ((path-rx (make-regexp
+                  (string-append"^" (regexp-quote (%store-prefix))
+                                "/([0-9a-df-np-sv-z]{32})-[^/]+$"))))
+    (and=> (regexp-exec path-rx path)
+           (cut match:substring <> 1))))