--- /dev/null
+! Copyright (c) 2009 Aaron Schaefer.
+! See http://factorcode.org/license.txt for BSD license.
+USING: fry kernel math math.primes math.ranges project-euler.common sequences ;
+IN: project-euler.058
+
+! http://projecteuler.net/index.php?section=problems&id=58
+
+! DESCRIPTION
+! -----------
+
+! Starting with 1 and solveling anticlockwise in the following way, a square
+! solve with side length 7 is formed.
+
+! 37 36 35 34 33 32 31
+! 38 17 16 15 14 13 30
+! 39 18 5 4 3 12 29
+! 40 19 6 1 2 11 28
+! 41 20 7 8 9 10 27
+! 42 21 22 23 24 25 26
+! 43 44 45 46 47 48 49
+
+! It is interesting to note that the odd squares lie along the bottom right
+! diagonal, but what is more interesting is that 8 out of the 13 numbers lying
+! along both diagonals are prime; that is, a ratio of 8/13 ≈ 62%.
+
+! If one complete new layer is wrapped around the solve above, a square solve
+! with side length 9 will be formed. If this process is continued, what is the
+! side length of the square solve for which the ratio of primes along both
+! diagonals first falls below 10%?
+
+
+! SOLUTION
+! --------
+
+<PRIVATE
+
+CONSTANT: PERCENT_PRIME 0.1
+
+! The corners of a square of side length n are:
+! (n-2)² + 1(n-1)
+! (n-2)² + 2(n-1)
+! (n-2)² + 3(n-1)
+! (n-2)² + 4(n-1) = odd squares, no need to calculate
+
+: prime-corners ( n -- m )
+ 3 [1,b] swap '[ _ [ 1- * ] keep 2 - sq + prime? ] count ;
+
+: total-corners ( n -- m )
+ 1- 2 * ; foldable
+
+: ratio-below? ( count length -- ? )
+ total-corners 1+ / PERCENT_PRIME < ;
+
+: next-layer ( count length -- count' length' )
+ 2 + [ prime-corners + ] keep ;
+
+: solve ( count length -- length )
+ 2dup ratio-below? [ nip ] [ next-layer solve ] if ;
+
+PRIVATE>
+
+: euler058 ( -- answer )
+ 8 7 solve ;
+
+! [ euler058 ] 10 ave-time
+! 12974 ms ave run time - 284.46 SD (10 trials)
+
+SOLUTION: euler058
project-euler.041 project-euler.042 project-euler.043 project-euler.044
project-euler.045 project-euler.046 project-euler.047 project-euler.048
project-euler.049 project-euler.052 project-euler.053 project-euler.054
- project-euler.055 project-euler.056 project-euler.057 project-euler.059
- project-euler.067 project-euler.071 project-euler.073 project-euler.075
- project-euler.076 project-euler.079 project-euler.092 project-euler.097
- project-euler.099 project-euler.100 project-euler.116 project-euler.117
- project-euler.134 project-euler.148 project-euler.150 project-euler.151
- project-euler.164 project-euler.169 project-euler.173 project-euler.175
- project-euler.186 project-euler.190 project-euler.203 project-euler.215 ;
+ project-euler.055 project-euler.056 project-euler.057 project-euler.058
+ project-euler.059 project-euler.067 project-euler.071 project-euler.073
+ project-euler.075 project-euler.076 project-euler.079 project-euler.092
+ project-euler.097 project-euler.099 project-euler.100 project-euler.116
+ project-euler.117 project-euler.134 project-euler.148 project-euler.150
+ project-euler.151 project-euler.164 project-euler.169 project-euler.173
+ project-euler.175 project-euler.186 project-euler.190 project-euler.203
+ project-euler.215 ;
IN: project-euler
<PRIVATE