Fix #3: Qty.toPrec returns wrong result with some precision

Bug reported by Adam Abrams

Readme.md

@@ -113,12 +113,12 @@ rounding issues.
 
 ### Rounding
 
-Qty#toPrec(precision) with precision as number, string or quantity standing for the minimum significative quantity.
-Returns a new quantity.
+Qty#toPrec(precision) : returns the nearest multiple of quantity passed as precision
 
  var qty = new Qty('5.17 ft');
  qty.toPrec('ft'); // => 5 ft
  qty.toPrec('0.5 ft'); // => 5 ft
+ qty.toPrec('0.25 ft'); // => 5.25 ft
  qty.toPrec('0.1 ft'); // => 5.2 ft
  qty.toPrec('0.05 ft'); // => 5.15 ft
  qty.toPrec('0.01 ft'); // => 5.17 ft
@@ -134,6 +134,9 @@ Returns a new quantity.
  qty.toPrec('10 m'); // => 10 m
  qty.toPrec(0.1); // => 6.3 m
 
+ var qty = new Qty('1.146 MPa');
+ qty.toPrec('0.1 bar'); // => 1.15 MPa
+
 ### Text output
 
  // if target_units string passed, the unit will first be converted to target_units before output.

spec/quantitiesSpec.js

@@ -950,6 +950,7 @@ describe('js-quantities', function() {
  expect(qty.toPrec(new Qty('0.05 ft')).toString()).toBe("5.15 ft");
  expect(qty.toPrec(new Qty('0.01 ft')).toString()).toBe("5.17 ft");
  expect(qty.toPrec(new Qty('0.0001 ft')).toString()).toBe("5.17 ft");
+ expect(qty.toPrec(new Qty('0.25 ft')).toString()).toBe("5.25 ft");
  });
 
  it('should allow string as precision parameter', function() {
@@ -976,11 +977,13 @@ describe('js-quantities', function() {
  expect(qty.toPrec(new Qty('0.01 MPa')).toString()).toBe("1.15 MPa");
  expect(qty.toPrec(new Qty('dbar')).toString()).toBe("1.15 MPa");
 
+ // Tests below are mainly a safety net because not sure if there is
+ // any usefulness to do things like that
  qty = new Qty('5.171234568 ft');
- expect(qty.toPrec(new Qty('m')).toString()).toBe("7 ft");
- expect(qty.toPrec(new Qty('dm')).toString()).toBe("5.2 ft");
- expect(qty.toPrec(new Qty('cm')).toString()).toBe("5.18 ft");
- expect(qty.toPrec(new Qty('mm')).toString()).toBe("5.171 ft");
+ expect(qty.toPrec(new Qty('m')).toString()).toBe("6.561679790026246 ft");
+ expect(qty.toPrec(new Qty('dm')).toString()).toBe("5.249343832020998 ft");
+ expect(qty.toPrec(new Qty('cm')).toString()).toBe("5.183727034120736 ft");
+ expect(qty.toPrec(new Qty('mm')).toString()).toBe("5.170603674540682 ft");
  });
  });
 

src/quantities.js

@@ -722,12 +722,28 @@ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLI
  // return a new quantity with same units rounded
  // at the nearest value according to quantity passed
  // as precision
+ /**
+ * Returns the nearest multiple of quantity passed as
+ * precision
+ *
+ * @param {(Qty|string|number)} prec-quantity - Quantity, string formated
+ * quantity or number as expected precision
+ *
+ * @returns {Qty} Nearest multiple of prec_quantity
+ *
+ * @example
+ * new Qty('5.5 ft').toPrec('2 ft'); // returns 6 ft
+ * new Qty('0.8 cu').toPrec('0.25 cu'); // returns 0.75 cu
+ * new Qty('6.3782 m').toPrec('cm'); // returns 6.38 m
+ * new Qty('1.146 MPa').toPrec('0.1 bar'); // returns 1.15 MPa
+ *
+ */
  toPrec: function(prec_quantity) {
  if(prec_quantity && prec_quantity.constructor === String) {
  prec_quantity = new Qty(prec_quantity);
  }
  if(typeof prec_quantity === "number") {
- prec_quantity = new Qty(prec_quantity+' '+this.units());
+ prec_quantity = new Qty(prec_quantity + ' ' + this.units());
  }
 
  if(!this.isUnitless()) {
@@ -739,11 +755,8 @@ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLI
 
  if(prec_quantity.scalar === 0)
  throw "Divide by zero";
- var prec_rounded_result = Math.round(this.scalar/prec_quantity.scalar)*prec_quantity.scalar;
-
- // Remove potential floating error based on prec_quantity exponent
- var prec_exponent = exponent_of(prec_quantity.scalar);
- prec_rounded_result = round(prec_rounded_result, -prec_exponent);
+ var prec_rounded_result = mul_safe(Math.round(this.scalar/prec_quantity.scalar),
+ prec_quantity.scalar);
 
  return new Qty(prec_rounded_result + this.units());
  },