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Viewing / Editing File: fuzzy.js

module.exports = function (text, pattern, options) {
  // Aproximately where in the text is the pattern expected to be found?
  var Match_Location = options.location || 0

//Determines how close the match must be to the fuzzy location (specified above). An exact letter match which is 'distance' characters away from the fuzzy location would score as a complete mismatch. A distance of '0' requires the match be at the exact location specified, a threshold of '1000' would require a perfect match to be within 800 characters of the fuzzy location to be found using a 0.8 threshold.
  var Match_Distance = options.distance || 100

// At what point does the match algorithm give up. A threshold of '0.0' requires a perfect match (of both letters and location), a threshold of '1.0' would match anything.
  var Match_Threshold = options.threshold || 0.4

if (pattern === text) return true // Exact match
  if (pattern.length > 32) return false // This algorithm cannot be used

// Set starting location at beginning text and initialise the alphabet.
  var loc = Match_Location,
    s = (function () {
      var q = {},
        i

for (i = 0; i < pattern.length; i++) {
        q[pattern.charAt(i)] = 0
      }

for (i = 0; i < pattern.length; i++) {
        q[pattern.charAt(i)] |= 1 << (pattern.length - i - 1)
      }

return q
    })()

// Compute and return the score for a match with e errors and x location.
  // Accesses loc and pattern through being a closure.

function match_bitapScore_(e, x) {
    var accuracy = e / pattern.length,
      proximity = Math.abs(loc - x)

if (!Match_Distance) {
      // Dodge divide by zero error.
      return proximity ? 1.0 : accuracy
    }
    return accuracy + proximity / Match_Distance
  }

var score_threshold = Match_Threshold, // Highest score beyond which we give up.
    best_loc = text.indexOf(pattern, loc) // Is there a nearby exact match? (speedup)

if (best_loc != -1) {
    score_threshold = Math.min(match_bitapScore_(0, best_loc), score_threshold)
    // What about in the other direction? (speedup)
    best_loc = text.lastIndexOf(pattern, loc + pattern.length)

if (best_loc != -1) {
      score_threshold = Math.min(match_bitapScore_(0, best_loc), score_threshold)
    }
  }

// Initialise the bit arrays.
  var matchmask = 1 << (pattern.length - 1)
  best_loc = -1

var bin_min, bin_mid
  var bin_max = pattern.length + text.length
  var last_rd
  for (var d = 0; d < pattern.length; d++) {
    // Scan for the best match; each iteration allows for one more error.
    // Run a binary search to determine how far from 'loc' we can stray at this
    // error level.
    bin_min = 0
    bin_mid = bin_max
    while (bin_min < bin_mid) {
      if (match_bitapScore_(d, loc + bin_mid) <= score_threshold) {
        bin_min = bin_mid
      } else {
        bin_max = bin_mid
      }
      bin_mid = Math.floor((bin_max - bin_min) / 2 + bin_min)
    }
    // Use the result from this iteration as the maximum for the next.
    bin_max = bin_mid
    var start = Math.max(1, loc - bin_mid + 1)
    var finish = Math.min(loc + bin_mid, text.length) + pattern.length

var rd = Array(finish + 2)
    rd[finish + 1] = (1 << d) - 1
    for (var j = finish; j >= start; j--) {
      // The alphabet (s) is a sparse hash, so the following line generates
      // warnings.
      var charMatch = s[text.charAt(j - 1)]
      if (d === 0) {
        // First pass: exact match.
        rd[j] = ((rd[j + 1] << 1) | 1) & charMatch
      } else {
        // Subsequent passes: fuzzy match.
        rd[j] = (((rd[j + 1] << 1) | 1) & charMatch) | (((last_rd[j + 1] | last_rd[j]) << 1) | 1) | last_rd[j + 1]
      }
      if (rd[j] & matchmask) {
        var score = match_bitapScore_(d, j - 1)
        // This match will almost certainly be better than any existing match.
        // But check anyway.
        if (score <= score_threshold) {
          // Told you so.
          score_threshold = score
          best_loc = j - 1
          if (best_loc > loc) {
            // When passing loc, don't exceed our current distance from loc.
            start = Math.max(1, 2 * loc - best_loc)
          } else {
            // Already passed loc, downhill from here on in.
            break
          }
        }
      }
    }
    // No hope for a (better) match at greater error levels.
    if (match_bitapScore_(d + 1, loc) > score_threshold) {
      break
    }
    last_rd = rd
  }

return best_loc < 0 ? false : true
}

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