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The current string shrink algorithm generates candidates by dropping single characters from the end of the input. This doesn't produce the smallest case, since it doesn't drop characters from the start of the string. It is also linear on the size of the input, so it requires a potentially large number of tries to reach the result it does produce. This PR changes to algorithm to bisect the input string from both directions, producing a minimal output in log n tries. As an example of the current behavior, the test: `forAll { it: String -> !it.contains("#") }` produces output like this: ``` Attempting to shrink failed arg DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k<}QQxU+!) e|Gr+ tri7jw{ Shrink #1: <empty string> pass Shrink #2: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k<}QQxU+!) e|Gr+ tri7 fail Shrink #3: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k<}QQxU+!) e|Gr+ fail Shrink #4: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k<}QQxU+!) fail Shrink #5: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k<}QQx fail Shrink #6: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x,L;6&k fail Shrink #7: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B*w,1x, fail Shrink #8: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn18+B* fail Shrink #9: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2yEcNn fail Shrink #10: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M#EC\M2 fail Shrink #11: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M# fail Shrink #12: DPn CPQ7hBAY&LP;7MxPtN^Oy\$ pass Shrink #13: DPn CPQ7hBAY&LP;7MxPtN^Oy\$F pass Shrink #14: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz pass Shrink #15: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz; pass Shrink #16: DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M pass Shrink #17: aaaaaDPn CPQ7hBAY&LP;7MxPtN^Oy\$ pass Shrink #18: aaaaDPn CPQ7hBAY&LP;7MxPtN^Oy\$F pass Shrink #19: aaaDPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz pass Shrink #20: aaDPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz; pass Shrink #21: aDPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M pass Shrink result => DPn CPQ7hBAY&LP;7MxPtN^Oy\$Fz;M# ``` This is 21 attempts to produce a string 32x longer than optimal. The new algorithm produces output like this: ``` Attempting to shrink failed arg t ^j>t\o,x3?eb9#F'>g>vGQ-N}nkx Shrink #1: <empty string> pass Shrink #2: t ^j>t\o,x3?eb9 pass Shrink #3: #F'>g>vGQ-N}nkx fail Shrink #4: #F'>g>vG fail Shrink #5: #F'> fail Shrink #6: #F fail Shrink #7: # fail Shrink result => # ``` This is only 7 tries, and produces the correct output of `"#"`
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