From 199d363b79e769bf8dc150bf37228395bfbb4d16 Mon Sep 17 00:00:00 2001
From: boblied <boblied@gmail.com>
Date: Fri, 5 Mar 2021 06:47:55 -0600
Subject: PWC 102 Task 1, Rare Numbers, in C

---
 challenge-102/bob-lied/c/ch-1.c | 82 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 challenge-102/bob-lied/c/ch-1.c

diff --git a/challenge-102/bob-lied/c/ch-1.c b/challenge-102/bob-lied/c/ch-1.c
new file mode 100644
index 0000000000..1ff39d59ec
--- /dev/null
+++ b/challenge-102/bob-lied/c/ch-1.c
@@ -0,0 +1,82 @@
+/* vim:set ts=4 sw=4 sts=4 et ai wm=0 nu syntax=c: */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+int verbose = 1;
+
+int mightBeRare[10]   = { 1, 0, 1, 1, 0, 1, 0, 1, 1, 0 };
+
+int mightBeSquare[10] = { 1, 1, 0, 0, 1, 1, 1, 0, 0, 1 };
+
+long
+reverse(long r)
+{
+    long r1 = 0;
+    while ( r )
+    {
+        r1 = r1 * 10 + r%10;
+        r /= 10;
+    }
+    return r1;
+}
+
+
+int
+main(int argc, char **argv)
+{
+    int N;
+
+    N = atoi(argv[1]);
+
+    int isNodd = N % 2;
+
+    long scale = pow(10, (N-1));
+    long endOfRange = pow(10, N);
+
+    for ( long r = scale ; r < endOfRange ; r++ )
+    {
+        // Rare numbers can never start with an odd digit.
+        if ( (r / scale ) % 2 )
+        {
+            r += scale;
+        }
+
+// if ( r % 10000000 == 0 ) printf("%ld\n", r);
+
+        if ( ! mightBeRare[ r % 10 ] )
+        {
+            continue;
+        }
+
+        long r1 = reverse(r);
+
+        long y2 = r - r1;
+        if ( y2 < 0 ) continue; // Can't be a square
+
+        long x2 = r + r1;
+
+        if ( !( mightBeSquare[ x2%10]  &&  mightBeSquare[ y2%10 ] ) )  continue;
+
+        if ( isNodd )
+        {
+            if ( y2 % 1089 ) continue;
+        }
+        else
+        {
+            if ( x2 % 121 ) continue;
+        }
+
+        double x = sqrt(x2);
+        if ( x != (long)(x) ) continue;
+
+        double y = sqrt(y2);
+        if ( y != (long)(y) ) continue;
+
+        printf("N=%d R=%ld\n", N, r);
+    }
+
+
+    exit(0);
+}
-- 
cgit