Amount: Full demurrage support.

src/js/ripple/amount.js

@@ -847,9 +847,11 @@ Amount.prototype.to_text = function (allow_nan) {
  *   default: 3.
  * @param opts.signed {Boolean|String} Whether negative numbers will have a
  *   prefix. If String, that string will be used as the prefix. Default: '-'
+ * @param opts.reference_date {Date|Number} Date based on which demurrage/interest
+ *   should be applied. Can be given as JavaScript Date or int for Ripple epoch.
  */
 Amount.prototype.to_human = function (opts) {
-  var opts = opts || {};
+  opts = opts || {};
 
   if (!this.is_valid()) return '';
 
@@ -859,10 +861,29 @@ Amount.prototype.to_human = function (opts) {
 
   opts.group_width = opts.group_width || 3;
 
-  var order         = this._is_native ? consts.xns_precision : -this._offset;
+  // Apply demurrage/interest
+  var ref = this;
+  if (opts.reference_date && this._currency.has_interest()) {
+    var interest = this._currency.get_interest_at(opts.reference_date);
+
+    // XXX Because the Amount parsing routines don't support some of the things
+    //     that JavaScript can output when casting a float to a string, the
+    //     following call sometimes does not produce a valid Amount.
+    //
+    //     The correct way to solve this is probably to switch to a proper
+    //     BigDecimal for our internal representation and then use that across
+    //     the board instead of instantiating these dummy Amount objects.
+    var interestTempAmount = Amount.from_json(""+interest+"/1/1");
+
+    if (interestTempAmount.is_valid()) {
+      ref = this.multiply(interestTempAmount);
+    }
+  }
+
+  var order         = ref._is_native ? consts.xns_precision : -ref._offset;
   var denominator   = consts.bi_10.clone().pow(order);
-  var int_part      = this._value.divide(denominator).toString();
-  var fraction_part = this._value.mod(denominator).toString();
+  var int_part      = ref._value.divide(denominator).toString();
+  var fraction_part = ref._value.mod(denominator).toString();
 
   // Add leading zeros to fraction
   while (fraction_part.length < order) {

src/js/ripple/currency.js

@@ -2,6 +2,7 @@
 var extend    = require('extend');
 
 var UInt160 = require('./uint160').UInt160;
+var Float = require('./float').Float;
 var utils = require('./utils');
 
 //
@@ -18,6 +19,8 @@ var Currency = extend(function () {
   // XXX Should support hex, C++ doesn't currently allow it.
 
   this._value  = NaN;
+
+  this._update();
 }, UInt160);
 
 Currency.prototype = extend({}, UInt160.prototype);
@@ -66,6 +69,7 @@ Currency.prototype.parse_json = function (j, shouldInterpretXrpAsIou) {
     case 'object':
       if (j instanceof Currency) {
         this._value = j.copyTo({})._value;
+        this._update();
       }
       break;
   }
@@ -73,6 +77,56 @@ Currency.prototype.parse_json = function (j, shouldInterpretXrpAsIou) {
   return this;
 };
 
+/**
+ * Recalculate internal representation.
+ *
+ * You should never need to call this.
+ */
+Currency.prototype._update = function () {
+  var bytes = this.to_bytes();
+
+  // is it 0 everywhere except 12, 13, 14?
+  var isZeroExceptInStandardPositions = true;
+
+  if (!bytes) {
+    return "XRP";
+  }
+
+  this._native = false;
+  this._type = -1;
+  this._interest_start = new Date();
+  this._interest_period = NaN;
+  this._iso_code = '';
+
+  for (var i=0; i<20; i++) {
+    isZeroExceptInStandardPositions = isZeroExceptInStandardPositions && (i===12 || i===13 || i===14 || bytes[i]===0);
+  }
+
+  if (isZeroExceptInStandardPositions) {
+    this._iso_code = String.fromCharCode(bytes[12])
+                   + String.fromCharCode(bytes[13])
+                   + String.fromCharCode(bytes[14]);
+
+    if (this._iso_code === "\0\0\0") {
+      this._native = true;
+      this._iso_code = "XRP";
+    }
+
+    this._type = 0;
+  } else if (bytes[0] === 0x01) { // Demurrage currency
+    this._iso_code = String.fromCharCode(bytes[1])
+                   + String.fromCharCode(bytes[2])
+                   + String.fromCharCode(bytes[3]);
+
+    this._type = 1;
+    this._interest_start = (bytes[4] << 24) +
+                           (bytes[5] << 16) +
+                           (bytes[6] <<  8) +
+                           (bytes[7]      );
+    this._interest_period = Float.fromBytes(bytes.slice(8, 16));
+  }
+};
+
 // XXX Probably not needed anymore?
 /*
 Currency.prototype.parse_bytes = function (byte_array) {
@@ -108,7 +162,24 @@ Currency.prototype.parse_bytes = function (byte_array) {
 */
 
 Currency.prototype.is_native = function () {
-  return !isNaN(this._value) && this.is_zero();
+  return this._native;
+};
+
+/**
+ * Whether this currency is an interest-bearing/demurring currency.
+ */
+Currency.prototype.has_interest = function () {
+  return this._type === 1 && this._interest_start && !isNaN(this._interest_period);
+};
+
+Currency.prototype.get_interest_at = function (referenceDate) {
+  if (!this.has_interest) return 1;
+
+  if (referenceDate instanceof Date) {
+    referenceDate = utils.fromTimestamp(referenceDate.getTime());
+  }
+
+  return Math.pow(Math.E, (referenceDate - this._interest_start) / this._interest_period);
 };
 
 // XXX Currently we inherit UInt.prototype.is_valid, which is mostly fine.
@@ -121,42 +192,26 @@ Currency.prototype.is_native = function () {
 //};
 
 Currency.prototype.to_json = function () {
-  var bytes = this.to_bytes();
-
-  // is it 0 everywhere except 12, 13, 14?
-  var isZeroExceptInStandardPositions = true;
-
-  if (!bytes) {
+  if (!this.is_valid()) {
+    // XXX This backwards compatible behavior, but probably not very good.
     return "XRP";
   }
 
-  for (var i=0; i<20; i++) {
-    isZeroExceptInStandardPositions = isZeroExceptInStandardPositions && (i===12 || i===13 || i===14 || bytes[i]===0);
+  if (/^[A-Z0-9]{3}$/.test(this._iso_code)) {
+    return this._iso_code;
   }
 
-  if (isZeroExceptInStandardPositions) {
-    var currencyCode = String.fromCharCode(bytes[12])
-                     + String.fromCharCode(bytes[13])
-                     + String.fromCharCode(bytes[14]);
-    if (/^[A-Z0-9]{3}$/.test(currencyCode) && currencyCode !== "XRP" ) {
-      return currencyCode;
-    } else if (currencyCode === "\0\0\0") {
-      return "XRP";
-    } else {
-      return "XRP";
-    }
-  } else {
-    var currencyHex = this.to_hex();
+  // Fallback to returning the raw currency hex
+  var currencyHex = this.to_hex();
 
-    // XXX This is to maintain backwards compatibility, but it is very, very odd
-    //     behavior, so we should deprecate it and get rid of it as soon as
-    //     possible.
-    if (currencyHex === Currency.HEX_ONE) {
-      return 1;
-    }
-
-    return currencyHex;
+  // XXX This is to maintain backwards compatibility, but it is very, very odd
+  //     behavior, so we should deprecate it and get rid of it as soon as
+  //     possible.
+  if (currencyHex === Currency.HEX_ONE) {
+    return 1;
   }
+
+  return currencyHex;
 };
 
 Currency.prototype.to_human = function () {

src/js/ripple/float.js

@@ -0,0 +1,56 @@
+/**
+ * IEEE 754 floating-point.
+ *
+ * Supports single- or double-precision
+ */
+var Float = exports.Float = {};
+
+var allZeros = /^0+$/;
+var allOnes = /^1+$/;
+
+Float.fromBytes = function (bytes) {
+  // Render in binary.  Hackish.
+  var b = "";
+  for (var i = 0, n = bytes.length; i < n; i++) {
+    var bits = (bytes[i] & 0xff).toString(2);
+    while (bits.length < 8) bits = "0" + bits;
+    b += bits;
+  }
+
+  // Determine configuration.  This could have all been precomputed but it is fast enough.
+  var exponentBits = bytes.length === 4 ? 4 : 11;
+  var mantissaBits = (bytes.length * 8) - exponentBits - 1;
+  var bias = Math.pow(2, exponentBits - 1) - 1;
+  var minExponent = 1 - bias - mantissaBits;
+
+  // Break up the binary representation into its pieces for easier processing.
+  var s = b[0];
+  var e = b.substring(1, exponentBits + 1);
+  var m = b.substring(exponentBits + 1);
+
+  var value = 0;
+  var multiplier = (s === "0" ? 1 : -1);
+
+  if (allZeros.test(e)) {
+    // Zero or denormalized
+    if (allZeros.test(m)) {
+      // Value is zero
+    } else {
+      value = parseInt(m, 2) * Math.pow(2, minExponent);
+    }
+  } else if (allOnes.test(e)) {
+    // Infinity or NaN
+    if (allZeros.test(m)) {
+      value = Infinity;
+    } else {
+      value = NaN;
+    }
+  } else {
+    // Normalized
+    var exponent = parseInt(e, 2) - bias;
+    var mantissa = parseInt(m, 2);
+    value = (1 + (mantissa * Math.pow(2, -mantissaBits))) * Math.pow(2, exponent);
+  }
+
+  return value * multiplier;
+};

test/currency-test.js

@@ -10,6 +10,9 @@ describe('Currency', function() {
     it('json_rewrite("NaN") == "XRP"', function() {
       assert.strictEqual('XRP', currency.json_rewrite(NaN));
     });
+    it('json_rewrite("015841551A748AD2C1F76FF6ECB0CCCD00000000") == "XAU"', function() {
+      assert.strictEqual('XAU', currency.json_rewrite("015841551A748AD2C1F76FF6ECB0CCCD00000000"));
+    });
   });
   describe('from_json', function() {
     it('from_json(NaN).to_json() == "XRP"', function() {
@@ -52,4 +55,37 @@ describe('Currency', function() {
       assert.strictEqual('XRP', currency.from_json('XRP').to_human());
     });
   });
+  describe('has_interest', function() {
+    it('should be true for type 1 currency codes', function() {
+      assert(currency.from_hex('015841551A748AD2C1F76FF6ECB0CCCD00000000').has_interest());
+      assert(currency.from_json('015841551A748AD2C1F76FF6ECB0CCCD00000000').has_interest());
+    });
+    it('should be false for type 0 currency codes', function() {
+      assert(!currency.from_hex('0000000000000000000000005553440000000000').has_interest());
+      assert(!currency.from_json('USD').has_interest());
+    });
+  });
+  function precision(num, precision) {
+    return +(Math.round(num + "e+"+precision)  + "e-"+precision);
+  }
+  describe('get_interest_at', function() {
+    it('returns demurred value for demurrage currency', function() {
+      var cur = currency.from_json('015841551A748AD2C1F76FF6ECB0CCCD00000000');
+
+      // At start, no demurrage should occur
+      assert.equal(1, cur.get_interest_at(443845330));
+
+      // After one year, 0.5% should have occurred
+      assert.equal(0.995, precision(cur.get_interest_at(443845330 + 31536000), 14));
+
+      // After one demurrage period, 1/e should have occurred
+      assert.equal(1/Math.E, cur.get_interest_at(443845330 + 6291418827.05));
+
+      // One year before start, it should be (roughly) 0.5% higher.
+      assert.equal(1.005, precision(cur.get_interest_at(443845330 - 31536000), 4));
+
+      // One demurrage period before start, rate should be e
+      assert.equal(Math.E, cur.get_interest_at(443845330 - 6291418827.05));
+    });
+  });
 });