commit d7a1b070427cde3d5b5a833434588b49d3bd471b
Author: Ray Miller <ray@metail.co.uk>
Date:   Thu Jul 4 20:20:55 2024 +0100

    Initial check-in.

diff --git a/geohash.scm b/geohash.scm
new file mode 100644
index 0000000..4cb7d5a
--- /dev/null
+++ b/geohash.scm
@@ -0,0 +1,129 @@
+;; A Geohash implementation for Guile scheme
+;; This is a port of the code from https://github.com/chrisveness/latlon-geohash/tree/master
+
+(define-module (anarres geohash)
+  #:use-module (ice-9 receive)
+  #:use-module (ice-9 match)
+  #:export (encode decode bounds adjacent neighbours))
+
+(define max-precision 12)
+
+(define base32 "0123456789bcdefghjkmnpqrstuvwxyz")
+
+(define* (encode lat lon #:key (precision 12))
+  (define desired-precision (min precision max-precision))
+  (let loop ((lat-min -90.0)
+             (lat-max 90.0)
+             (lon-min -180.0)
+             (lon-max 180.0)
+             (even-bit? #t)
+             (idx 0)
+             (bit 0)
+             (result '()))
+    (cond
+     ((= desired-precision (length result))
+      (reverse-list->string result))
+
+     ((= bit 5)
+      (loop lat-min lat-max lon-min lon-max even-bit?
+            0 0 (cons (string-ref base32 idx) result)))
+
+     (even-bit?
+      (let ((lon-mid (/ (+ lon-min lon-max) 2)))
+        (if (>= lon lon-mid)
+            (loop lat-min lat-max lon-mid lon-max (not even-bit?)
+                  (1+ (* 2 idx)) (1+ bit) result)
+            (loop lat-min lat-max lon-min lon-mid (not even-bit?)
+                  (* 2 idx) (1+ bit) result))))
+
+     (else
+      (let ((lat-mid (/ (+ lat-min lat-max) 2)))
+        (if (>= lat lat-mid)
+            (loop lat-mid lat-max lon-min lon-max (not even-bit?)
+                  (1+ (* 2 idx)) (1+ bit) result)
+            (loop lat-min lat-mid lon-min lon-max (not even-bit?)
+                  (* 2 idx) (1+ bit) result)))))))
+
+(define (bounds s)
+  (let loop ((xs (map (lambda (c) (string-index base32 c)) (string->list s)))
+             (n 4)
+             (lat-min -90.0)
+             (lat-max 90.0)
+             (lon-min -180.0)
+             (lon-max 180.0)
+             (even-bit? #t))
+    (cond
+     ((nil? xs)
+      (values lat-min lat-max lon-min lon-max))
+
+     ((< n 0)
+      (loop (cdr xs) 4 lat-min lat-max lon-min lon-max even-bit?))
+
+     (even-bit?
+      (let ((lon-mid (/ (+ lon-min lon-max) 2)))
+        (if (logbit? n (car xs))
+            (loop xs (1- n) lat-min lat-max lon-mid lon-max (not even-bit?))
+            (loop xs (1- n) lat-min lat-max lon-min lon-mid (not even-bit?)))))
+
+     (else
+      (let ((lat-mid (/ (+ lat-min lat-max) 2)))
+        (if (logbit? n (car xs))
+            (loop xs (1- n) lat-mid lat-max lon-min lon-max (not even-bit?))
+            (loop xs (1- n) lat-min lat-mid lon-min lon-max (not even-bit?))))))))
+
+(define (round-to-n-decimal-places x n)
+  (let* ((whole-part (truncate x))
+         (fractional-part (- x whole-part))
+         (multiplier (expt 10 n))
+         (rounded-frac (round (* fractional-part multiplier))))
+    (+ whole-part (/ rounded-frac multiplier))))
+
+(define (decode s)
+  (receive (lat-min lat-max lon-min lon-max) (bounds s)
+    (let ((lat (/ (+ lat-min lat-max) 2))
+          (lat-places (truncate (- 2 (log10 (- lat-max lat-min)))))
+          (lon (/ (+ lon-min lon-max) 2))
+          (lon-places (truncate (- 2 (log10 (- lon-max lon-min))))))
+      (values (round-to-n-decimal-places lat lat-places)
+              (round-to-n-decimal-places lon lon-places)))))
+
+(define (neighbour direction geohash-length)
+  (match (list direction (even? geohash-length))
+    (('n #t) "p0r21436x8zb9dcf5h7kjnmqesgutwvy")
+    (('n #f) "bc01fg45238967deuvhjyznpkmstqrwx")
+    (('s #t) "14365h7k9dcfesgujnmqp0r2twvyx8zb")
+    (('s #f) "238967debc01fg45kmstqrwxuvhjyznp")
+    (('e #t) "bc01fg45238967deuvhjyznpkmstqrwx")
+    (('e #f) "p0r21436x8zb9dcf5h7kjnmqesgutwvy")
+    (('w #t) "238967debc01fg45kmstqrwxuvhjyznp")
+    (('w #f) "14365h7k9dcfesgujnmqp0r2twvyx8zb")))
+
+(define (border direction geohash-length)
+  (match (list direction (even? geohash-length))
+    (('n #t) "prxz")
+    (('n #f) "bcfguvyz")
+    (('s #t) "028b")
+    (('s #f) "0145hjnp")
+    (('e #t) "bcfguvyz")
+    (('e #f) "prxz")
+    (('w #t) "0145hjnp")
+    (('w #f) "028b")))
+
+(define (adjacent s direction)
+  (let* ((n (string-length s))
+         (last-char (string-ref s (1- n)))
+         (nbr-idx (string-index (neighbour direction n) last-char))
+         (parent (if (and (> n 1) (string-index (border direction n) last-char))
+                     (adjacent (substring s 0 (1- n)) direction)
+                     (substring s 0 (1- n)))))
+    (string-append parent (string (string-ref base32 nbr-idx)))))
+
+(define (neighbours s)
+  (list (cons 'n (adjacent s 'n))
+        (cons 'ne (adjacent (adjacent s 'n) 'e))
+        (cons 'e (adjacent s 'e))
+        (cons 'se (adjacent (adjacent s 's) 'e))
+        (cons 's (adjacent s 's))
+        (cons 'sw (adjacent (adjacent s 's) 'w))
+        (cons 'w (adjacent s 'w))
+        (cons 'nw (adjacent (adjacent s 'n) 'w))))