Add balance retrieval of NEP5 tokens

app/api/crypto/nep5.js

@@ -0,0 +1,51 @@
+import { isValidPublicAddress } from './index'
+import bs58 from 'bs58'
+import { reverse } from './utils'
+
+/**
+ * Get the raw script hex string for use in an invokescript rpc call
+ * @param {String} token hash (hex)
+ * @param {String} public address of account to check token balance of
+ * @returns {Buffer} raw script data
+ */
+export function getTokenBalanceScript(token, address) {
+    if (isValidPublicAddress(address)) {
+        let programHashBuffer = bs58.decode(address)
+        let scriptHash = programHashBuffer.slice(1, 21)
+
+        const SCRIPTHASH_LEN = 0x14
+        const OPCODE_PUSH1 = 0x51
+        const OPCODE_PACK = 0xc1
+        const OPERATION_LENGTH = 9
+        const OPERATION = Buffer.from('balanceOf', 'utf8')
+        const OPCODE_APPCALL = 0x67
+        let token_scripthash_buffer = reverse(Buffer.from(token, 'hex'))
+
+        // construct script
+        let offset = 0
+        let data = Buffer.alloc(54)
+
+        data.writeUInt8(SCRIPTHASH_LEN, offset)
+        offset += 1
+
+        data.fill(scriptHash, offset, offset + scriptHash.length)
+        offset += scriptHash.length
+
+        data.writeUInt8(OPCODE_PUSH1, offset)
+        data.writeUInt8(OPCODE_PACK, offset + 1)
+        data.writeUInt8(OPERATION_LENGTH, offset + 2)
+        offset += 3
+
+        data.fill(OPERATION, offset, offset + OPERATION.length)
+        offset += OPERATION.length
+
+        data.writeUInt8(OPCODE_APPCALL, offset)
+        offset += 1
+
+        data.fill(token_scripthash_buffer, offset)
+
+        return data
+    } else {
+        throw new Error('Invalid public address')
+    }
+}

app/api/network/index.js

@@ -6,6 +6,8 @@ import {
     buildRawTransaction,
     signTransactionData
 } from '../crypto'
+import { getTokenBalanceScript } from '../crypto/nep5'
+import { reverse } from '../crypto/utils'
 
 export function getBalance(address) {
     var path = '/v2/address/balance/' + address
@@ -186,3 +188,23 @@ export function getWalletDataFrom(url) {
         }
     })
 }
+
+/**
+ * Get the balance of a NEP5 Token
+ * @param {String} token hash (hex)
+ * @param {String} public address of account to check token balance of
+ * @returns {int} token abalance
+ */
+
+export function getTokenBalance(token, address) {
+    const NETWORK_STORAGE_MULTIPLIER = 100000000
+    return queryRPC('invokescript', [getTokenBalanceScript(token, address).toString('hex')], 2).then(response => {
+        let valueBuf = Buffer.from(response.result.stack[0].value, 'hex')
+        let value = parseInt(reverse(valueBuf).toString('hex'), 16) / NETWORK_STORAGE_MULTIPLIER
+
+        if (isNaN(value)) {
+            value = 0
+        }
+        return value
+    })
+}