knothash stuff

knothash/CMakeLists.txt

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+add_library(knothash src/knothash.cpp)
+target_include_directories(knothash PUBLIC include)
+target_link_libraries(knothash PRIVATE aocpp)

knothash/include/knothash.hpp

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+#ifndef KNOTHASH_HPP_
+#define KNOTHASH_HPP_
+
+#include <cstddef>
+#include <array>
+#include <algorithm>
+#include <string>
+
+namespace knothash {
+
+template <std::size_t N>
+auto reverser(std::array<std::uint8_t, N> & result, std::size_t cursor, std::size_t len) -> void {
+  auto leftside = N - cursor;
+  auto start = result.begin() + cursor;
+  if (len <= leftside) {
+    std::reverse(start, start + len);
+  } else {
+    auto rightside = len - leftside;
+    if (leftside < rightside) {
+      std::swap_ranges(start, result.end(), result.rend() - rightside);
+      std::reverse(result.begin(), result.begin() + (rightside-leftside));
+    } else {
+      std::swap_ranges(result.rend() - rightside, result.rend(), start);
+      std::reverse(result.end() - (leftside-rightside), result.end());
+    }
+  }
+}
+
+auto hash(std::string key) -> std::array<std::uint8_t, 16>;
+auto render(std::array<std::uint8_t, 16> const& hash) -> std::string;
+
+template <std::size_t N, typename It>
+auto hash_ex(std::size_t cycles, It begin, It end)
+ -> std::array<std::uint8_t, N>
+{
+  std::size_t cursor = 0;
+  std::size_t skip = 0;
+  
+  std::array<std::uint8_t, N> result;
+  for (std::size_t i = 0; i < N; i++) { result[i] = i; }
+
+  for (std::size_t i = 0; i < cycles; i++) {
+    std::for_each(begin, end, [&](std::size_t len) {
+      reverser(result, cursor, len);
+      cursor += skip + len;
+      cursor %= N;
+      skip++;
+    });
+  }
+  return result;
+}
+
+} // namespace
+
+#endif

knothash/src/knothash.cpp

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+#include <knothash.hpp>
+
+#include <numeric>
+#include <iomanip>
+#include <sstream>
+
+#include <doctest.h>
+
+namespace knothash {
+
+auto hash(std::string key) -> std::array<std::uint8_t, 16>
+{
+  key += 17;
+  key += 31;
+  key += 73;
+  key += 47;
+  key += 23;
+  auto sparse = hash_ex<256>(64, key.begin(), key.end());
+
+  std::size_t offset = 0;
+  std::array<std::uint8_t, 16> dense;
+  for (auto & x : dense) {
+    x = std::reduce(sparse.begin()+offset, sparse.begin()+offset+16, 0, std::bit_xor());
+    offset += 16;
+  } 
+
+  return dense;
+}
+
+auto render(std::array<std::uint8_t, 16> const& hash) -> std::string {
+  std::ostringstream out;
+  out << std::hex << std::setfill('0');
+  for (int const x : hash) {
+    out << std::setw(2) << int(x);
+  }
+  return out.str();
+}
+
+TEST_SUITE("knot hash") {
+
+TEST_CASE("reverser") {
+  std::array<std::uint8_t, 9> v {0,1,2,3,4,5,6,7,8};
+
+  SUBCASE("more left") {
+    std::array<std::uint8_t, 9> expect {1,0,8,7,4,5,6,3,2};
+    reverser(v, 7, 6);
+    REQUIRE(v == expect);
+  }
+
+  SUBCASE("more right") {
+    std::array<std::uint8_t, 9> expect {6,5,2,3,4,1,0,8,7};
+    reverser(v, 5, 6);
+    REQUIRE(v == expect);
+  }
+
+  SUBCASE("middle") {
+    std::array<std::uint8_t, 9> expect {0,1,5,4,3,2,6,7,8};
+    reverser(v, 2, 4);
+    CHECK(v == expect);
+  }
+}
+
+TEST_CASE("knot example") {
+  std::array<std::size_t, 4> lengths {3, 4, 1, 5};
+  auto result = hash_ex<5>(1, lengths.begin(), lengths.end());
+  std::array<std::uint8_t, 5> expected {3, 4, 2, 1, 0};
+  CHECK(result == expected);
+}
+
+TEST_CASE("hash examples") {
+  CHECK(render(hash(""))         == "a2582a3a0e66e6e86e3812dcb672a272");
+  CHECK(render(hash("AoC 2017")) == "33efeb34ea91902bb2f59c9920caa6cd");
+  CHECK(render(hash("1,2,3"))    == "3efbe78a8d82f29979031a4aa0b16a9d");
+  CHECK(render(hash("1,2,4"))    == "63960835bcdc130f0b66d7ff4f6a5a8e");
+}
+
+}
+
+} // namespace