index.js

var wordWrap = require('word-wrapper')
var xtend = require('xtend')
var number = require('as-number')

var X_HEIGHTS = ['x', 'e', 'a', 'o', 'n', 's', 'r', 'c', 'u', 'm', 'v', 'w', 'z']
var M_WIDTHS = ['m', 'w']
var CAP_HEIGHTS = ['H', 'I', 'N', 'E', 'F', 'K', 'L', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']


var TAB_ID = '\t'.charCodeAt(0)
var SPACE_ID = ' '.charCodeAt(0)
var ALIGN_LEFT = 0, 
    ALIGN_CENTER = 1, 
    ALIGN_RIGHT = 2

module.exports = function createLayout(opt) {
  return new TextLayout(opt)
}

function TextLayout(opt) {
  this.glyphs = []
  this._measure = this.computeMetrics.bind(this)
  this.update(opt)
}

TextLayout.prototype.update = function(opt) {
  opt = xtend({
    measure: this._measure
  }, opt)
  this._opt = opt
  this._opt.tabSize = number(this._opt.tabSize, 4)

  if (!opt.font)
    throw new Error('must provide a valid bitmap font')

  var glyphs = this.glyphs
  var text = opt.text||'' 
  var font = opt.font
  this._setupSpaceGlyphs(font)
  
  var lines = wordWrap.lines(text, opt)
  var minWidth = opt.width || 0

  //clear glyphs
  glyphs.length = 0

  //get max line width
  var maxLineWidth = lines.reduce(function(prev, line) {
    return Math.max(prev, line.width, minWidth)
  }, 0)

  //the pen position
  var x = 0
  var y = 0
  var lineHeight = number(opt.lineHeight, font.common.lineHeight)
  var baseline = font.common.base
  var descender = lineHeight-baseline
  var letterSpacing = opt.letterSpacing || 0
  var height = lineHeight * lines.length - descender
  var align = getAlignType(this._opt.align)

  //draw text along baseline
  y -= height
  
  //the metrics for this text layout
  this._width = maxLineWidth
  this._height = height
  this._descender = lineHeight - baseline
  this._baseline = baseline
  this._xHeight = getXHeight(font)
  this._capHeight = getCapHeight(font)
  this._lineHeight = lineHeight
  this._ascender = lineHeight - descender - this._xHeight
    
  //layout each glyph
  var self = this
  lines.forEach(function(line, lineIndex) {
    var start = line.start
    var end = line.end
    var lineWidth = line.width
    var lastGlyph
    
    //for each glyph in that line...
    for (var i=start; i<end; i++) {
      var id = text.charCodeAt(i)
      var glyph = self.getGlyph(font, id)
      if (glyph) {
        if (lastGlyph) 
          x += getKerning(font, lastGlyph.id, glyph.id)

        var tx = x
        if (align === ALIGN_CENTER) 
          tx += (maxLineWidth-lineWidth)/2
        else if (align === ALIGN_RIGHT)
          tx += (maxLineWidth-lineWidth)

        glyphs.push({
          position: [tx, y],
          data: glyph,
          index: i,
          line: lineIndex
        })  

        //move pen forward
        x += glyph.xadvance + letterSpacing
        lastGlyph = glyph
      }
    }

    //next line down
    y += lineHeight
    x = 0
  })
  this._linesTotal = lines.length;
}

TextLayout.prototype._setupSpaceGlyphs = function(font) {
  //These are fallbacks, when the font doesn't include
  //' ' or '\t' glyphs
  this._fallbackSpaceGlyph = null
  this._fallbackTabGlyph = null

  if (!font.chars || font.chars.length === 0)
    return

  //try to get space glyph
  //then fall back to the 'm' or 'w' glyphs
  //then fall back to the first glyph available
  var space = getGlyphById(font, SPACE_ID) 
          || getMGlyph(font) 
          || font.chars[0]

  //and create a fallback for tab
  var tabWidth = this._opt.tabSize * space.xadvance
  this._fallbackSpaceGlyph = space
  this._fallbackTabGlyph = xtend(space, {
    x: 0, y: 0, xadvance: tabWidth, id: TAB_ID, 
    xoffset: 0, yoffset: 0, width: 0, height: 0
  })
}

TextLayout.prototype.getGlyph = function(font, id) {
  var glyph = getGlyphById(font, id)
  if (glyph)
    return glyph
  else if (id === TAB_ID) 
    return this._fallbackTabGlyph
  else if (id === SPACE_ID) 
    return this._fallbackSpaceGlyph
  return null
}

TextLayout.prototype.computeMetrics = function(text, start, end, width) {
  var letterSpacing = this._opt.letterSpacing || 0
  var font = this._opt.font
  var curPen = 0
  var curWidth = 0
  var count = 0
  var glyph
  var lastGlyph

  if (!font.chars || font.chars.length === 0) {
    return {
      start: start,
      end: start,
      width: 0
    }
  }

  end = Math.min(text.length, end)
  for (var i=start; i < end; i++) {
    var id = text.charCodeAt(i)
    var glyph = this.getGlyph(font, id)

    if (glyph) {
      //move pen forward
      var xoff = glyph.xoffset
      var kern = lastGlyph ? getKerning(font, lastGlyph.id, glyph.id) : 0
      curPen += kern

      var nextPen = curPen + glyph.xadvance + letterSpacing
      var nextWidth = curPen + glyph.width

      //we've hit our limit; we can't move onto the next glyph
      if (nextWidth >= width || nextPen >= width)
        break

      //otherwise continue along our line
      curPen = nextPen
      curWidth = nextWidth
      lastGlyph = glyph
    }
    count++
  }
  
  //make sure rightmost edge lines up with rendered glyphs
  if (lastGlyph)
    curWidth += lastGlyph.xoffset

  return {
    start: start,
    end: start + count,
    width: curWidth
  }
}

//getters for the private vars
;['width', 'height', 
  'descender', 'ascender',
  'xHeight', 'baseline',
  'capHeight',
  'lineHeight' ].forEach(addGetter)

function addGetter(name) {
  Object.defineProperty(TextLayout.prototype, name, {
    get: wrapper(name),
    configurable: true
  })
}

//create lookups for private vars
function wrapper(name) {
  return (new Function([
    'return function '+name+'() {',
    '  return this._'+name,
    '}'
  ].join('\n')))()
}

function getGlyphById(font, id) {
  if (!font.chars || font.chars.length === 0)
    return null

  var glyphIdx = findChar(font.chars, id)
  if (glyphIdx >= 0)
    return font.chars[glyphIdx]
  return null
}

function getXHeight(font) {
  for (var i=0; i<X_HEIGHTS.length; i++) {
    var id = X_HEIGHTS[i].charCodeAt(0)
    var idx = findChar(font.chars, id)
    if (idx >= 0) 
      return font.chars[idx].height
  }
  return 0
}

function getMGlyph(font) {
  for (var i=0; i<M_WIDTHS.length; i++) {
    var id = M_WIDTHS[i].charCodeAt(0)
    var idx = findChar(font.chars, id)
    if (idx >= 0) 
      return font.chars[idx]
  }
  return 0
}

function getCapHeight(font) {
  for (var i=0; i<CAP_HEIGHTS.length; i++) {
    var id = CAP_HEIGHTS[i].charCodeAt(0)
    var idx = findChar(font.chars, id)
    if (idx >= 0) 
      return font.chars[idx].height
  }
  return 0
}

function getKerning(font, left, right) {
  if (!font.kernings || font.kernings.length === 0)
    return 0

  var table = font.kernings
  for (var i=0; i<table.length; i++) {
    var kern = table[i]
    if (kern.first === left && kern.second === right)
      return kern.amount
  }
  return 0
}

function getAlignType(align) {
  if (align === 'center')
    return ALIGN_CENTER
  else if (align === 'right')
    return ALIGN_RIGHT
  return ALIGN_LEFT
}

function findChar (array, value, start) {
  start = start || 0
  for (var i = start; i < array.length; i++) {
    if (array[i].id === value) {
      return i
    }
  }
  return -1
}