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authorLudovic Courtès <ludo@gnu.org>2021-09-09 23:22:10 +0200
committerLudovic Courtès <ludo@gnu.org>2021-09-10 17:30:54 +0200
commitcb06f7c61e4b8393abf38f1f5891e03c33d53b9b (patch)
tree5ae33fcbe4a683a03cc7149caa819ad4bdbee4da
parenta87d8c912d64382d8d7489c156249bc2b2638df0 (diff)
base32: Provide an open-coded 'bit-field'.
This improves the throughput of 'bytevector->base32-string' a bit. * guix/base32.scm (bit-field): New macro.
-rw-r--r--guix/base32.scm15
1 files changed, 14 insertions, 1 deletions
diff --git a/guix/base32.scm b/guix/base32.scm
index 49f191ba26..d6c8a02243 100644
--- a/guix/base32.scm
+++ b/guix/base32.scm
@@ -1,5 +1,5 @@
;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2015, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2015, 2017, 2021 Ludovic Courtès <ludo@gnu.org>
;;;
;;; This file is part of GNU Guix.
;;;
@@ -42,6 +42,19 @@
;;;
;;; Code:
+(define-syntax bit-field
+ (lambda (s)
+ ;; This inline version of 'bit-field' assumes that START and END are
+ ;; literals and pre-computes the mask. In an ideal world, using 'define'
+ ;; or 'define-inlinable' would be enough, but as of 3.0.7, peval doesn't
+ ;; expand calls to 'expt' (and 'bit-field' is a subr.)
+ (syntax-case s ()
+ ((_ n start end)
+ (let* ((s (syntax->datum #'start))
+ (e (syntax->datum #'end))
+ (mask (- (expt 2 (- e s)) 1)))
+ #`(logand (ash n (- start)) #,mask))))))
+
(define bytevector-quintet-ref
(let* ((ref bytevector-u8-ref)
(ref+ (lambda (bv offset)