Add asmap utility which queries a mapping

Summary:
```
The scripts for creating a compact IP->ASN mapping are here:
https://github.com/sipa/asmap
```

Partial backport 1/4 of core [[bitcoin/bitcoin#16702 | PR16702]]:
bitcoin/bitcoin@8feb4e4

Test Plan:
  ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8194

src/CMakeLists.txt

@@ -343,6 +343,7 @@ add_library(util
 	sync.cpp
 	threadinterrupt.cpp
 	uint256.cpp
+	util/asmap.cpp
 	util/bip32.cpp
 	util/bytevectorhash.cpp
 	util/error.cpp

src/util/asmap.cpp

@@ -0,0 +1,96 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <crypto/common.h>
+
+#include <cassert>
+#include <vector>
+
+namespace {
+
+uint32_t DecodeBits(std::vector<bool>::const_iterator &bitpos, uint8_t minval,
+                    const std::vector<uint8_t> &bit_sizes) {
+    uint32_t val = minval;
+    bool bit;
+    for (std::vector<uint8_t>::const_iterator bit_sizes_it = bit_sizes.begin();
+         bit_sizes_it != bit_sizes.end(); ++bit_sizes_it) {
+        if (bit_sizes_it + 1 != bit_sizes.end()) {
+            bit = *bitpos;
+            bitpos++;
+        } else {
+            bit = 0;
+        }
+        if (bit) {
+            val += (1 << *bit_sizes_it);
+        } else {
+            for (int b = 0; b < *bit_sizes_it; b++) {
+                bit = *bitpos;
+                bitpos++;
+                val += bit << (*bit_sizes_it - 1 - b);
+            }
+            return val;
+        }
+    }
+    return -1;
+}
+
+const std::vector<uint8_t> TYPE_BIT_SIZES{0, 0, 1};
+uint32_t DecodeType(std::vector<bool>::const_iterator &bitpos) {
+    return DecodeBits(bitpos, 0, TYPE_BIT_SIZES);
+}
+
+const std::vector<uint8_t> ASN_BIT_SIZES{15, 16, 17, 18, 19,
+                                         20, 21, 22, 23, 24};
+uint32_t DecodeASN(std::vector<bool>::const_iterator &bitpos) {
+    return DecodeBits(bitpos, 1, ASN_BIT_SIZES);
+}
+
+const std::vector<uint8_t> MATCH_BIT_SIZES{1, 2, 3, 4, 5, 6, 7, 8};
+uint32_t DecodeMatch(std::vector<bool>::const_iterator &bitpos) {
+    return DecodeBits(bitpos, 2, MATCH_BIT_SIZES);
+}
+
+const std::vector<uint8_t> JUMP_BIT_SIZES{5,  6,  7,  8,  9,  10, 11, 12, 13,
+                                          14, 15, 16, 17, 18, 19, 20, 21, 22,
+                                          23, 24, 25, 26, 27, 28, 29, 30};
+uint32_t DecodeJump(std::vector<bool>::const_iterator &bitpos) {
+    return DecodeBits(bitpos, 17, JUMP_BIT_SIZES);
+}
+
+} // namespace
+
+uint32_t Interpret(const std::vector<bool> &asmap,
+                   const std::vector<bool> &ip) {
+    std::vector<bool>::const_iterator pos = asmap.begin();
+    uint8_t bits = ip.size();
+    uint8_t default_asn = 0;
+    uint32_t opcode, jump, match, matchlen;
+    while (1) {
+        assert(pos != asmap.end());
+        opcode = DecodeType(pos);
+        if (opcode == 0) {
+            return DecodeASN(pos);
+        } else if (opcode == 1) {
+            jump = DecodeJump(pos);
+            if (ip[ip.size() - bits]) {
+                pos += jump;
+            }
+            bits--;
+        } else if (opcode == 2) {
+            match = DecodeMatch(pos);
+            matchlen = CountBits(match) - 1;
+            for (uint32_t bit = 0; bit < matchlen; bit++) {
+                if ((ip[ip.size() - bits]) !=
+                    ((match >> (matchlen - 1 - bit)) & 1)) {
+                    return default_asn;
+                }
+                bits--;
+            }
+        } else if (opcode == 3) {
+            default_asn = DecodeASN(pos);
+        } else {
+            assert(0);
+        }
+    }
+}

src/util/asmap.h

@@ -0,0 +1,10 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_ASMAP_H
+#define BITCOIN_UTIL_ASMAP_H
+
+uint32_t Interpret(const std::vector<bool> &asmap, const std::vector<bool> &ip);
+
+#endif // BITCOIN_UTIL_ASMAP_H