gistfile1.txt

/* 
                         /-------------\
                        /               \
                       /                 \
                      /                   \
                      |   XXXX     XXXX   |
                      |   XXXX     XXXX   |
                      |   XXX       XXX   |
                      \         X         /
                       --\     XXX     /--
                        | |    XXX    | |
                        | |           | |
                        | I I I I I I I |
                        |  I I I I I I  |
                         \             /
                          --         --
                            \-------/
                    XXX                    XXX
                   XXXXX                  XXXXX
                   XXXXXXXXX         XXXXXXXXXX
                          XXXXX   XXXXX
                             XXXXXXX
                          XXXXX   XXXXX
                   XXXXXXXXX         XXXXXXXXXX
                   XXXXX                  XXXXX
                    XXX                    XXX

  ************************************************************
  *  BEWARE!!  IF YOU THINK YOU UNDERSTAND THIS, YOU DO NOT. *
  ************************************************************
*/
 exports.isTween = function(l, p) {
    return (l[0][0] == l[1][0]
        ? (Math.min(l[0][1], l[1][1]) <= p[1] 
            && p[1] <= Math.max(l[0][1], l[1][1])
          ? (l[0][1] == l[1][1] ? 0 : (p[1] - l[0][1]) / (l[1][1] - l[0][1]))
          : null)
        : (l[0][1] == l[1][1] 
          ? (Math.min(l[0][0], l[1][0]) <= p[0] 
              && p[0] <= Math.max(l[0][0], l[1][0])
            ? (p[0] - l[0][0]) / (l[1][0] - l[0][0])
            : null)
          : ((p[1] - l[0][1]) / (l[1][1] - l[0][1]) 
              == (p[0] - l[0][0]) / (l[1][0] - l[0][0])
            ? (p[0] - l[0][0]) / (l[1][0] - l[0][0])
            : null)));
  };