Script: generate prefix for base58 encoding

scripts/b58_prefix.py

@@ -0,0 +1,44 @@
+#! /usr/bin/env python
+
+import sys
+import bitcoin
+
+alphabet = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'
+
+
+def b58dec(word):
+    x = 0
+    for c in word:
+        x *= 58
+        x += alphabet.find(c)
+    return x
+
+def asciidec(val):
+    word = []
+    while val > 0:
+        word.append(val % 256)
+        val /= 256
+    return word[-1::-1]
+
+if __name__ == '__main__':
+
+    prefix = sys.argv[1]
+    length = int(sys.argv[2])
+    target = b58dec(prefix)
+
+    shift = 8*(length+4)
+
+    for m in range(1,1000):
+        lo = target * 58**m
+        lo = (lo >> shift) + (0 if lo == ((lo >> shift) << shift) else 1)
+        hi = (target + 1) * 58**m - (1 << shift) +1
+        hi = hi >> shift
+        if hi >= lo:
+            # test
+            for bt in '\x00\xff':
+                s = bitcoin.bin_to_b58check(bt * length, magicbyte=lo)
+                assert s.startswith(prefix)
+                assert len(s) == m + len(prefix)
+
+            print m + len(prefix), lo, asciidec(lo)
+            exit(0)