first commit

intersection.js

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+exports.intersection = function (line1StartX, line1StartY, line1EndX, line1EndY, line2StartX, line2StartY, line2EndX, line2EndY) {
+    // if the lines intersect, the result contains the x and y of the intersection (treating the lines as infinite) and booleans for whether line segment 1 or line segment 2 contain the point
+    var denominator, a, b, numerator1, numerator2, result = {
+        x: null,
+        y: null,
+        onLine1: false,
+        onLine2: false
+    };
+    denominator = ((line2EndY - line2StartY) * (line1EndX - line1StartX)) - ((line2EndX - line2StartX) * (line1EndY - line1StartY));
+    if (Math.abs(denominator) < 0.000000000001) {
+        return result;
+    }
+    a = line1StartY - line2StartY;
+    b = line1StartX - line2StartX;
+    numerator1 = ((line2EndX - line2StartX) * a) - ((line2EndY - line2StartY) * b);
+    numerator2 = ((line1EndX - line1StartX) * a) - ((line1EndY - line1StartY) * b);
+    a = numerator1 / denominator;
+    b = numerator2 / denominator;
+
+    // if we cast these lines infinitely in both directions, they intersect here:
+    result.x = line1StartX + (a * (line1EndX - line1StartX));
+    result.y = line1StartY + (a * (line1EndY - line1StartY));
+/*
+        // it is worth noting that this should be the same as:
+        x = line2StartX + (b * (line2EndX - line2StartX));
+        y = line2StartX + (b * (line2EndY - line2StartY));
+        */
+    // if line1 is a segment and line2 is infinite, they intersect if:
+    if (a > 0 && a < 1) {
+        result.onLine1 = true;
+    }
+    // if line2 is a segment and line1 is infinite, they intersect if:
+    if (b > 0 && b < 1) {
+        result.onLine2 = true;
+    }
+    // if line1 and line2 are segments, they intersect if both of the above are true
+    return result;
+};