'use strict'


module.exports = kerning


var canvas = kerning.canvas = document.createElement('canvas')
var ctx = canvas.getContext('2d')
var asciiPairs = createPairs([32, 126])

kerning.createPairs = createPairs
kerning.ascii = asciiPairs


function kerning (family, o) {
	if (Array.isArray(family)) family = family.join(', ')

	var table = {}, pairs, fs = 16, threshold = .05

	if (o) {
		if (o.length === 2 && typeof o[0] === 'number') {
			pairs = createPairs(o)
		}
		else if (Array.isArray(o)) {
			pairs = o
		}
		else {
			if (o.o) pairs = createPairs(o.o)
			else if (o.pairs) pairs = o.pairs

			if (o.fontSize) fs = o.fontSize
			if (o.threshold != null) threshold = o.threshold
		}
	}

	if (!pairs) pairs = asciiPairs

	ctx.font = fs + 'px ' + family

	for (var i = 0; i < pairs.length; i++) {
		var pair = pairs[i]
		var width = ctx.measureText(pair[0]).width + ctx.measureText(pair[1]).width
		var kerningWidth = ctx.measureText(pair).width
		if (Math.abs(width - kerningWidth) > fs * threshold) {
			var emWidth = (kerningWidth - width) / fs
			table[pair] = emWidth * 1000
		}
	}

	return table
}


function createPairs (range) {
	var pairs = []

    for (var i = range[0]; i <= range[1]; i++) {
		var leftChar = String.fromCharCode(i)
		for (var j = range[0]; j < range[1]; j++) {
			var rightChar = String.fromCharCode(j)
			var pair = leftChar + rightChar

			pairs.push(pair)
		}
	}

	return pairs
}