Solution to task 2

1 files changed, 117 insertions, 0 deletions

diff --git a/challenge-101/jo-37/perl/ch-2.pl b/challenge-101/jo-37/perl/ch-2.pl
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+#!/usr/bin/perl -s
+
+use v5.16;
+use PDL;
+use List::Util 'pairs';
+use Test2::V0 '!float';
+
+our ($tests, $examples, $verbose);
+
+run_tests() if $tests || $examples;	# does not return
+
+die <<EOS unless @ARGV == 6;
+usage: $0 [-examples] [-tests] [-verbose] [--] [x1 y1 x2 y2 x3 y3]
+
+-examples
+    run the examples from the challenge
+ 
+-tests
+    run some tests
+
+-verbose
+    enable trace output
+
+x1 ... y3
+    coordinates of the triangle's corners
+
+EOS
+
+
+### Input and Output
+
+say inner_origin(pairs @ARGV);
+
+
+### Implementation
+
+# Check if the origin [0, 0] is inside a given triangle or on its
+# border.  The triangle is specified by the coordinates of its corners
+# and may be degenerated: The corners may be located on a common line
+# and need not be distinct.
+# Two consecutive checks are performed:
+# 1) Origin orientation:
+#    The coordinates of each pair of the triangle's corners form a 2x2
+#    matrix.  The sign of the corresponding determinant signals if the
+#    origin is left or right of the directed edge connecting these
+#    points.  If the origin is an inner point, all three orientations
+#    must agree, whereas an outer point will show different signs.
+# 2) Axis projection:
+#    If the three given points are collinear, all determinants are zero
+#    and cannot be used as an indicator for "inner" and "outer".  In
+#    that case the projection of the points onto the x and y axes reveal
+#    the location of the origin inside or outside of the line segment.
+sub inner_origin {
+    # Convert coordinate pairs to a 3x2 piddle.
+    my $p = pdl(@_)->xchg(0,1)->sever;
+    say "coords:$p" if $verbose;
+
+    # Get the minimum and the maximum of the three matrices'
+    # determinants formed by the point pairs.
+    my ($min_d, $max_d) = $p->range([[0, 0], [1, 0], [2, 0]], 2, 'p')
+        ->reorder(1, 2, 0)->determinant->minmax;
+    say "min/max det: $min_d/$max_d" if $verbose;
+
+    # If determinants have different signs, the origin is outside the
+    # triangle.
+    return 0 if $min_d < 0 && $max_d > 0;
+
+    # If there is at least one nonzero determinant and there are no
+    # differing signs, the origin is located inside the triangle (or on
+    # its border).
+    return 1 if $min_d >= 0 && $max_d > 0 || $min_d < 0 && $max_d <= 0;
+
+    # At this point all determinants are zero.
+
+    # Get the projections onto the x and y axis for collinear points
+    # and check if they both contain the origin.
+    my ($min_p, $max_p) = $p->minmaximum;
+    say "min/max proj: $min_p/$max_p" if $verbose;
+
+    return 1 if max($min_p) <= 0 && min($max_p) >= 0;
+
+    # Else: origin is not within the given line segment.
+    0;
+}
+
+
+### Examples and tests
+
+sub run_tests {
+    SKIP: {
+        skip "examples" unless $examples;
+
+        is inner_origin([0, 1], [1, 0], [2, 2]), F(), 'example 1';
+        is inner_origin([1, 1], [-1, 1], [0,-3]), T(), 'example 2';
+        is inner_origin([0, 1], [2, 0], [-6, 0]), T(), 'example 3';
+    }
+
+    SKIP: {
+        skip "tests" unless $tests;
+
+        is inner_origin([-1, -1], [-1, 2], [2, -1]), T(), 'inside';
+        is inner_origin([2, -1], [2, 2], [-1, 2]), F(), 'outside';
+        is inner_origin([0, 0], [3, 1], [1, 2]), T(), 'origin at corner';
+        is inner_origin([1, 1], [3, 3], [-1, -2]), F(), 'two collinear points';
+        is inner_origin([-1, -2], [0, -1], [1, 0]), F(), 'flat';
+        is inner_origin([-1, -2], [1, 0], [1, 0]), F(), 'two points';
+        is inner_origin([-1, -1], [1, 1], [2, 2]), T(), 'aligned around origin';
+        is inner_origin([-1, 1], [-2, 2], [-4, 4]), F(),
+            'aligned without origin';
+        is inner_origin([0, 0], [0, 0], [1, 1]), T(), 'two points at origin';
+        is inner_origin([0, 0], [0, 0], [0, 0]), T(), 'single point at origin';
+        is inner_origin([1, 2], [1, 2], [1, 2]), F(), 'single point off origin';
+	}
+
+    done_testing;
+    exit;
+}