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Filename : alignment-pattern.js
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/**
 * Alignment pattern are fixed reference pattern in defined positions
 * in a matrix symbology, which enables the decode software to re-synchronise
 * the coordinate mapping of the image modules in the event of moderate amounts
 * of distortion of the image.
 *
 * Alignment patterns are present only in QR Code symbols of version 2 or larger
 * and their number depends on the symbol version.
 */

const getSymbolSize = require('./utils').getSymbolSize

/**
 * Calculate the row/column coordinates of the center module of each alignment pattern
 * for the specified QR Code version.
 *
 * The alignment patterns are positioned symmetrically on either side of the diagonal
 * running from the top left corner of the symbol to the bottom right corner.
 *
 * Since positions are simmetrical only half of the coordinates are returned.
 * Each item of the array will represent in turn the x and y coordinate.
 * @see {@link getPositions}
 *
 * @param  {Number} version QR Code version
 * @return {Array}          Array of coordinate
 */
exports.getRowColCoords = function getRowColCoords (version) {
  if (version === 1) return []

  const posCount = Math.floor(version / 7) + 2
  const size = getSymbolSize(version)
  const intervals = size === 145 ? 26 : Math.ceil((size - 13) / (2 * posCount - 2)) * 2
  const positions = [size - 7] // Last coord is always (size - 7)

  for (let i = 1; i < posCount - 1; i++) {
    positions[i] = positions[i - 1] - intervals
  }

  positions.push(6) // First coord is always 6

  return positions.reverse()
}

/**
 * Returns an array containing the positions of each alignment pattern.
 * Each array's element represent the center point of the pattern as (x, y) coordinates
 *
 * Coordinates are calculated expanding the row/column coordinates returned by {@link getRowColCoords}
 * and filtering out the items that overlaps with finder pattern
 *
 * @example
 * For a Version 7 symbol {@link getRowColCoords} returns values 6, 22 and 38.
 * The alignment patterns, therefore, are to be centered on (row, column)
 * positions (6,22), (22,6), (22,22), (22,38), (38,22), (38,38).
 * Note that the coordinates (6,6), (6,38), (38,6) are occupied by finder patterns
 * and are not therefore used for alignment patterns.
 *
 * let pos = getPositions(7)
 * // [[6,22], [22,6], [22,22], [22,38], [38,22], [38,38]]
 *
 * @param  {Number} version QR Code version
 * @return {Array}          Array of coordinates
 */
exports.getPositions = function getPositions (version) {
  const coords = []
  const pos = exports.getRowColCoords(version)
  const posLength = pos.length

  for (let i = 0; i < posLength; i++) {
    for (let j = 0; j < posLength; j++) {
      // Skip if position is occupied by finder patterns
      if ((i === 0 && j === 0) || // top-left
          (i === 0 && j === posLength - 1) || // bottom-left
          (i === posLength - 1 && j === 0)) { // top-right
        continue
      }

      coords.push([pos[i], pos[j]])
    }
  }

  return coords
}