Adds varIntToNum conversion function

src/convert.js

@@ -134,6 +134,28 @@ function numToVarInt(num) {
   return [255].concat(numToBytes(num, 8));
 }
 
+/**
+ * Turn an VarInt into an integer
+ *
+ * "var_int" is a variable length integer used by Bitcoin's binary format.
+ *
+ * Returns { bytes: bytesUsed, number: theNumber }
+ */
+function varIntToNum(bytes) {
+  var prefix = bytes[0]
+
+  var viBytes =
+      prefix < 253   ? bytes.slice(0, 1)
+    : prefix === 253 ? bytes.slice(1, 3)
+    : prefix === 254 ? bytes.slice(1, 5)
+                     : bytes.slice(1, 9)
+
+  return {
+    bytes: prefix < 253 ? viBytes : bytes.slice(0, viBytes.length + 1),
+    number: bytesToNum(viBytes)
+  }
+}
+
 function bytesToWords(bytes) {
   var words = [];
   for (var i = 0, b = 0; i < bytes.length; i++, b += 8) {
@@ -176,6 +198,7 @@ module.exports = {
   numToBytes: numToBytes,
   bytesToNum: bytesToNum,
   numToVarInt: numToVarInt,
+  varIntToNum: varIntToNum,
   bytesToWords: bytesToWords,
   wordsToBytes: wordsToBytes,
   bytesToWordArray: bytesToWordArray,

test/convert.js

@@ -94,6 +94,45 @@ describe('convert', function() {
       })
     })
 
+    describe('varIntToNum', function() {
+      it('works on valid input', function() {
+        var data = [
+          [0], [128], [252], // 8-bit
+          [253, 0, 1], [253, 0, 2], [253, 0, 4], // 16-bit
+          [254, 5, 0, 1, 0], // 32-bit
+          [255, 3, 0, 0, 0, 1, 0, 0, 0] // 64-bit
+        ]
+        var expected = [
+          0, 128, 252, // 8-bit
+          256, 512, 1024, // 16-bit
+          65541, // 32-bit
+          4294967299, // 64-bit
+        ]
+
+        for (var i = 0; i < data.length; ++i) {
+          var actual = convert.varIntToNum(data[i])
+          assert.equal(actual.number, expected[i])
+          assert.deepEqual(actual.bytes, data[i])
+        }
+      })
+
+      it('uses only what is necessary', function() {
+        var data = [
+          [0, 99],
+          [253, 0, 1, 99],
+          [254, 5, 0, 1, 0, 99],
+          [255, 3, 0, 0, 0, 1, 0, 0, 0, 99]
+        ]
+        var expected = [0, 256, 65541, 4294967299]
+
+        for (var i = 0; i < data.length; ++i) {
+          var actual = convert.varIntToNum(data[i])
+          assert.equal(actual.number, expected[i])
+          assert.deepEqual(actual.bytes, data[i].slice(0, -1))
+        }
+      })
+    })
+
     describe('reverseEndian', function() {
         it('works', function() {
             var bigEndian = "6a4062273ac4f9ea4ffca52d9fd102b08f6c32faa0a4d1318e3a7b2e437bb9c7"