module.exports = function (tree, unitConversions) {

//
// A number with a unit
//
var Dimension = function (value, unit) {
    this.value = parseFloat(value);
    this.unit = (unit && unit instanceof tree.Unit) ? unit :
      new(tree.Unit)(unit ? [unit] : undefined);
};

Dimension.prototype = {
    type: "Dimension",
    accept: function (visitor) {
        this.unit = visitor.visit(this.unit);
    },
    eval: function (env) {
        return this;
    },
    toColor: function () {
        return new(tree.Color)([this.value, this.value, this.value]);
    },
    genCSS: function (env, output) {
        if ((env && env.strictUnits) && !this.unit.isSingular()) {
            throw new Error("Multiple units in dimension. Correct the units or use the unit function. Bad unit: "+this.unit.toString());
        }

        var value = tree.fround(env, this.value),
            strValue = String(value);

        if (value !== 0 && value < 0.000001 && value > -0.000001) {
            // would be output 1e-6 etc.
            strValue = value.toFixed(20).replace(/0+$/, "");
        }

        if (env && env.compress) {
            // Zero values doesn't need a unit
            if (value === 0 && this.unit.isLength()) {
                output.add(strValue);
                return;
            }

            // Float values doesn't need a leading zero
            if (value > 0 && value < 1) {
                strValue = (strValue).substr(1);
            }
        }

        output.add(strValue);
        this.unit.genCSS(env, output);
    },
    toCSS: tree.toCSS,

    // In an operation between two Dimensions,
    // we default to the first Dimension's unit,
    // so `1px + 2` will yield `3px`.
    operate: function (env, op, other) {
        /*jshint noempty:false */
        var value = tree.operate(env, op, this.value, other.value),
            unit = this.unit.clone();

        if (op === '+' || op === '-') {
            if (unit.numerator.length === 0 && unit.denominator.length === 0) {
                unit.numerator = other.unit.numerator.slice(0);
                unit.denominator = other.unit.denominator.slice(0);
            } else if (other.unit.numerator.length === 0 && unit.denominator.length === 0) {
                // do nothing
            } else {
                other = other.convertTo(this.unit.usedUnits());

                if(env.strictUnits && other.unit.toString() !== unit.toString()) {
                  throw new Error("Incompatible units. Change the units or use the unit function. Bad units: '" + unit.toString() +
                    "' and '" + other.unit.toString() + "'.");
                }

                value = tree.operate(env, op, this.value, other.value);
            }
        } else if (op === '*') {
            unit.numerator = unit.numerator.concat(other.unit.numerator).sort();
            unit.denominator = unit.denominator.concat(other.unit.denominator).sort();
            unit.cancel();
        } else if (op === '/') {
            unit.numerator = unit.numerator.concat(other.unit.denominator).sort();
            unit.denominator = unit.denominator.concat(other.unit.numerator).sort();
            unit.cancel();
        }
        return new(Dimension)(value, unit);
    },

    compare: function (other) {
        if (other instanceof Dimension) {
            var a, b,
                aValue, bValue;
            
            if (this.unit.isEmpty() || other.unit.isEmpty()) {
                a = this;
                b = other;
            } else {
                a = this.unify();
                b = other.unify();
                if (a.unit.compare(b.unit) !== 0) {
                    return -1;
                }                
            }
            aValue = a.value;
            bValue = b.value;

            if (bValue > aValue) {
                return -1;
            } else if (bValue < aValue) {
                return 1;
            } else {
                return 0;
            }
        } else {
            return -1;
        }
    },

    unify: function () {
        return this.convertTo({ length: 'px', duration: 's', angle: 'rad' });
    },

    convertTo: function (conversions) {
        var value = this.value, unit = this.unit.clone(),
            i, groupName, group, targetUnit, derivedConversions = {}, applyUnit;

        if (typeof conversions === 'string') {
            for(i in unitConversions) {
                if (unitConversions[i].hasOwnProperty(conversions)) {
                    derivedConversions = {};
                    derivedConversions[i] = conversions;
                }
            }
            conversions = derivedConversions;
        }
        applyUnit = function (atomicUnit, denominator) {
          /*jshint loopfunc:true */
            if (group.hasOwnProperty(atomicUnit)) {
                if (denominator) {
                    value = value / (group[atomicUnit] / group[targetUnit]);
                } else {
                    value = value * (group[atomicUnit] / group[targetUnit]);
                }

                return targetUnit;
            }

            return atomicUnit;
        };

        for (groupName in conversions) {
            if (conversions.hasOwnProperty(groupName)) {
                targetUnit = conversions[groupName];
                group = unitConversions[groupName];

                unit.map(applyUnit);
            }
        }

        unit.cancel();

        return new(Dimension)(value, unit);
    }
};
return Dimension;
};