script_number: add impl/tests

src/script_number.js

@@ -0,0 +1,78 @@
+function decode (buffer, maxLength, minimal) {
+  maxLength = maxLength || 4
+  minimal = minimal === undefined ? true : minimal
+
+  var length = buffer.length
+  if (length === 0) return 0
+  if (length > maxLength) throw new TypeError('Script number overflow')
+  if (minimal) {
+    if ((buffer[length - 1] & 0x7f) === 0) {
+      if (length <= 1 || (buffer[length - 2] & 0x80) === 0) throw new Error('Non-minimally encoded script number')
+    }
+  }
+
+  // 32-bit?
+  if (length < 5) {
+    var result = 0
+
+    for (var i = 0; i < length; ++i) {
+      result += buffer[i] << (8 * i)
+    }
+
+    if (buffer[length - 1] & 0x80) return -(result & ~(0x80 << (8 * (length - 1))))
+    return result
+  }
+
+  // 40-bit
+  var a = buffer.readUInt32LE(0)
+  var b = buffer.readUInt8(4)
+
+  // TODO: refactor
+  var neg = false
+  if (b & 0x80) {
+    b &= ~0x80
+    neg = true
+  }
+
+  var r = b * 0x100000000 + a
+
+  if (neg) {
+    r = -r
+  }
+
+  return r
+}
+
+function scriptNumSize (i) {
+  return i > 0x7fffffff ? 5
+  : i > 0x7fffff ? 4
+  : i > 0x7fff ? 3
+  : i > 0x7f ? 2
+  : i > 0x00 ? 1
+  : 0
+}
+
+function encode (number) {
+  var value = Math.abs(number)
+  var size = scriptNumSize(value)
+  var buffer = new Buffer(size)
+  var negative = number < 0
+
+  for (var i = 0; i < size; ++i) {
+    buffer.writeUInt8(value & 0xff, i)
+    value >>= 8
+  }
+
+  if (buffer[size - 1] & 0x80) {
+    buffer.writeUInt8(negative ? 0x80 : 0x00, size - 1)
+  } else if (negative) {
+    buffer[size - 1] |= 0x80
+  }
+
+  return buffer
+}
+
+module.exports = {
+  decode: decode,
+  encode: encode
+}