// Copyright (c) 2019-present 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 <util/asmap.h>

#include <hash.h>

#include <streams.h>

#include <uint256.h>

#include <util/check.h>

#include <util/fs.h>

#include <util/log.h>

#include <bit>

#include <cstddef>

#include <cstdio>

#include <span>

#include <string>

#include <utility>

#include <vector>

/*

 * ASMap (Autonomous System Map) Implementation

 *

 * Provides a compressed mapping from IP address prefixes to Autonomous System Numbers (ASNs).

 * Uses a binary trie structure encoded as bytecode instructions that are interpreted

 * at runtime to find the ASN for a given IP address.

 *

 * The format of the asmap data is a bit-packed binary format where the entire mapping

 * is treated as a continuous sequence of bits. Instructions and their arguments are

 * encoded using variable numbers of bits and concatenated together without regard for

 * byte boundaries. The bits are stored in bytes using little-endian bit ordering.

 *

 * The data structure internally represents the mapping as a binary trie where:

 * - Unassigned subnets (no ASN mapping present) map to 0

 * - Subnets mapped entirely to one ASN become leaf nodes

 * - Subnets whose lower and upper halves have different mappings branch into subtrees

 *

 * The encoding uses variable-length integers and four instruction types (RETURN, JUMP,

 * MATCH, DEFAULT) to efficiently represent the trie.

 */

namespace {

// Indicates decoding errors or invalid data

constexpr uint32_t INVALID = 0xFFFFFFFF;

/**

 * Extract a single bit from byte array using little-endian bit ordering (LSB first).

 * Used for ASMap data.

 */

inline bool ConsumeBitLE(size_t& bitpos, std::span<const std::byte> bytes) noexcept

{

    const bool bit = (std::to_integer<uint8_t>(bytes[bitpos / 8]) >> (bitpos % 8)) & 1;

    ++bitpos;

    return bit;

}

/**

 * Extract a single bit from byte array using big-endian bit ordering (MSB first).

 * Used for IP addresses to match network byte order conventions.

 */

inline bool ConsumeBitBE(uint8_t& bitpos, std::span<const std::byte> bytes) noexcept

{

    const bool bit = (std::to_integer<uint8_t>(bytes[bitpos / 8]) >> (7 - (bitpos % 8))) & 1;

    ++bitpos;

    return bit;

}

/**

 * Variable-length integer decoder using a custom encoding scheme.

 *

 * The encoding is easiest to describe using an example. Let's say minval=100 and

 * bit_sizes=[4,2,2,3]. In that case:

 * - x in [100..115]: encoded as [0] + [4-bit BE encoding of (x-100)]

 * - x in [116..119]: encoded as [1,0] + [2-bit BE encoding of (x-116)]

 * - x in [120..123]: encoded as [1,1,0] + [2-bit BE encoding of (x-120)]

 * - x in [124..131]: encoded as [1,1,1] + [3-bit BE encoding of (x-124)]

 *

 * In general, every number is encoded as:

 * - First, k "1"-bits, where k is the class the number falls in

 * - Then, a "0"-bit, unless k is the highest class

 * - Lastly, bit_sizes[k] bits encoding in big endian the position within that class

 */

uint32_t DecodeBits(size_t& bitpos, const std::span<const std::byte> data, uint8_t minval, const std::span<const uint8_t> bit_sizes)

{

    uint32_t val = minval;  // Start with minimum encodable value

    bool bit;

    for (auto bit_sizes_it = bit_sizes.begin(); bit_sizes_it != bit_sizes.end(); ++bit_sizes_it) {

        // Read continuation bit to determine if we're in this class

        if (bit_sizes_it + 1 != bit_sizes.end()) {  // Unless we're in the last class

            if (bitpos >= data.size() * 8) break;

            bit = ConsumeBitLE(bitpos, data);

        } else {

            bit = 0;  // Last class has no continuation bit

        }

        if (bit) {

            // If the value will not fit in this class, subtract its range from val,

            // emit a "1" bit and continue with the next class

            val += (1 << *bit_sizes_it);  // Add size of this class

        } else {

            // Decode the position within this class in big endian

            for (int b = 0; b < *bit_sizes_it; b++) {

                if (bitpos >= data.size() * 8) return INVALID; // Reached EOF in mantissa

                bit = ConsumeBitLE(bitpos, data);

                val += bit << (*bit_sizes_it - 1 - b); // Big-endian within the class

            }

            return val;

        }

    }

    return INVALID; // Reached EOF in exponent

}

/**

 * Instruction Set

 *

 * The instruction set is designed to efficiently encode a binary trie

 * that maps IP prefixes to ASNs. Each instruction type serves a specific

 * role in trie traversal and evaluation.

 */

enum class Instruction : uint32_t

{

    // A return instruction, encoded as [0], returns a constant ASN.

    // It is followed by an integer using the ASN encoding.

    RETURN = 0,

    // A jump instruction, encoded as [1,0], inspects the next unused bit in the input

    // and either continues execution (if 0), or skips a specified number of bits (if 1).

    // It is followed by an integer using jump encoding.

    JUMP = 1,

    // A match instruction, encoded as [1,1,0], inspects 1 or more of the next unused bits

    // in the input. If they all match, execution continues. If not, the default ASN is returned

    // (or 0 if unset). The match value encodes both the pattern and its length.

    MATCH = 2,

    // A default instruction, encoded as [1,1,1], sets the default variable to its argument,

    // and continues execution. It is followed by an integer in ASN encoding.

    DEFAULT = 3,

};

// Instruction type encoding: RETURN=[0], JUMP=[1,0], MATCH=[1,1,0], DEFAULT=[1,1,1]

constexpr uint8_t TYPE_BIT_SIZES[]{0, 0, 1};

Instruction DecodeType(size_t& bitpos, const std::span<const std::byte> data)

{

    return Instruction(DecodeBits(bitpos, data, 0, TYPE_BIT_SIZES));

}

// ASN encoding: Can encode ASNs from 1 to ~16.7 million.

// Uses variable-length encoding optimized for real-world ASN distribution.

// ASN 0 is reserved and used if there isn't a match.

constexpr uint8_t ASN_BIT_SIZES[]{15, 16, 17, 18, 19, 20, 21, 22, 23, 24};

uint32_t DecodeASN(size_t& bitpos, const std::span<const std::byte> data)

{

    return DecodeBits(bitpos, data, 1, ASN_BIT_SIZES);

}

// MATCH argument: Values in [2, 511]. The highest set bit determines the match length

// n ∈ [1,8]; the lower n-1 bits are the pattern to compare.

constexpr uint8_t MATCH_BIT_SIZES[]{1, 2, 3, 4, 5, 6, 7, 8};

uint32_t DecodeMatch(size_t& bitpos, const std::span<const std::byte> data)

{

    return DecodeBits(bitpos, data, 2, MATCH_BIT_SIZES);

}

// JUMP offset: Minimum value 17. Variable-length coded and may be large

// for skipping big subtrees.

constexpr 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(size_t& bitpos, const std::span<const std::byte> data)

{

    return DecodeBits(bitpos, data, 17, JUMP_BIT_SIZES);

}

} // anonymous namespace

/**

 * Execute the ASMap bytecode to find the ASN for an IP

 *

 * This function interprets the asmap bytecode and uses bits from the IP

 * address to navigate through the encoded trie structure, ultimately

 * returning an ASN value.

 */

uint32_t Interpret(const std::span<const std::byte> asmap, const std::span<const std::byte> ip)

{

    size_t pos{0};

    const size_t endpos{asmap.size() * 8};

    uint8_t ip_bit{0};

    const uint8_t ip_bits_end = ip.size() * 8;

    uint32_t default_asn = 0;

    while (pos < endpos) {

        Instruction opcode = DecodeType(pos, asmap);

        if (opcode == Instruction::RETURN) {

            // Found leaf node - return the ASN

            uint32_t asn = DecodeASN(pos, asmap);

            if (asn == INVALID) break; // ASN straddles EOF

            return asn;

        } else if (opcode == Instruction::JUMP) {

            // Binary branch: if IP bit is 1, jump forward; else continue

            uint32_t jump = DecodeJump(pos, asmap);

            if (jump == INVALID) break; // Jump offset straddles EOF

            if (ip_bit == ip_bits_end) break; // No input bits left

            if (int64_t{jump} >= static_cast<int64_t>(endpos - pos)) break; // Jumping past EOF

            if (ConsumeBitBE(ip_bit, ip)) {  // Check next IP bit (big-endian)

                pos += jump;  // Bit = 1: skip to right subtree

            }

            // Bit = 0: fall through to left subtree

        } else if (opcode == Instruction::MATCH) {

            // Compare multiple IP bits against a pattern

            // The match value encodes both length and pattern:

            // - highest set bit position determines length (bit_width - 1)

            // - lower bits contain the pattern to compare

            uint32_t match = DecodeMatch(pos, asmap);

            if (match == INVALID) break; // Match bits straddle EOF

            int matchlen = std::bit_width(match) - 1;  // An n-bit value matches n-1 input bits

            if ((ip_bits_end - ip_bit) < matchlen) break; // Not enough input bits

            for (int bit = 0; bit < matchlen; bit++) {

                if (ConsumeBitBE(ip_bit, ip) != ((match >> (matchlen - 1 - bit)) & 1)) {

                    return default_asn;  // Pattern mismatch - use default

                }

            }

            // Pattern matched - continue execution

        } else if (opcode == Instruction::DEFAULT) {

            // Update the default ASN for subsequent MATCH failures

            default_asn = DecodeASN(pos, asmap);

            if (default_asn == INVALID) break; // ASN straddles EOF

        } else {

            break; // Instruction straddles EOF

        }

    }

    // Reached EOF without RETURN, or aborted (see any of the breaks above)

    // - should have been caught by SanityCheckAsmap below

    assert(false);

    return 0; // 0 is not a valid ASN

}

/**

 * Validates ASMap structure by simulating all possible execution paths.

 * Ensures well-formed bytecode, valid jumps, and proper termination.

 */

bool SanityCheckAsmap(const std::span<const std::byte> asmap, int bits)

{

    size_t pos{0};

    const size_t endpos{asmap.size() * 8};

    std::vector<std::pair<uint32_t, int>> jumps; // All future positions we may jump to (bit offset in asmap -> bits to consume left)

    jumps.reserve(bits);

    Instruction prevopcode = Instruction::JUMP;

    bool had_incomplete_match = false;  // Track <8 bit matches for efficiency check

    while (pos != endpos) {

        // There was a jump into the middle of the previous instruction

        if (!jumps.empty() && pos >= jumps.back().first) return false;

        Instruction opcode = DecodeType(pos, asmap);

        if (opcode == Instruction::RETURN) {

            // There should not be any RETURN immediately after a DEFAULT (could be combined into just RETURN)

            if (prevopcode == Instruction::DEFAULT) return false;

            uint32_t asn = DecodeASN(pos, asmap);

            if (asn == INVALID) return false; // ASN straddles EOF

            if (jumps.empty()) {

                // Nothing to execute anymore

                if (endpos - pos > 7) return false; // Excessive padding

                while (pos != endpos) {

                    if (ConsumeBitLE(pos, asmap)) return false; // Nonzero padding bit

                }

                return true; // Sanely reached EOF

            } else {

                // Continue by pretending we jumped to the next instruction

                if (pos != jumps.back().first) return false; // Unreachable code

                bits = jumps.back().second; // Restore the number of bits we would have had left after this jump

                jumps.pop_back();

                prevopcode = Instruction::JUMP;

            }

        } else if (opcode == Instruction::JUMP) {

            uint32_t jump = DecodeJump(pos, asmap);

            if (jump == INVALID) return false; // Jump offset straddles EOF

            if (int64_t{jump} > static_cast<int64_t>(endpos - pos)) return false; // Jump out of range

            if (bits == 0) return false; // Consuming bits past the end of the input

            --bits;

            uint32_t jump_offset = pos + jump;

            if (!jumps.empty() && jump_offset >= jumps.back().first) return false; // Intersecting jumps

            jumps.emplace_back(jump_offset, bits);  // Queue jump target for validation

            prevopcode = Instruction::JUMP;

        } else if (opcode == Instruction::MATCH) {

            uint32_t match = DecodeMatch(pos, asmap);

            if (match == INVALID) return false; // Match bits straddle EOF

            int matchlen = std::bit_width(match) - 1;

            if (prevopcode != Instruction::MATCH) had_incomplete_match = false;

            // Within a sequence of matches only at most one should be incomplete

            if (matchlen < 8 && had_incomplete_match) return false;

            had_incomplete_match = (matchlen < 8);

            if (bits < matchlen) return false; // Consuming bits past the end of the input

            bits -= matchlen;

            prevopcode = Instruction::MATCH;

        } else if (opcode == Instruction::DEFAULT) {

            // There should not be two successive DEFAULTs (they could be combined into one)

            if (prevopcode == Instruction::DEFAULT) return false;

            uint32_t asn = DecodeASN(pos, asmap);

            if (asn == INVALID) return false; // ASN straddles EOF

            prevopcode = Instruction::DEFAULT;

        } else {

            return false; // Instruction straddles EOF

        }

    }

    return false; // Reached EOF without RETURN instruction

}

/**

 * Provides a safe interface for validating ASMap data before use.

 * Returns true if the data is valid for 128 bits long inputs.

 */

bool CheckStandardAsmap(const std::span<const std::byte> data)

{

    if (!SanityCheckAsmap(data, 128)) {

        LogWarning("Sanity check of asmap data failed\n");

        return false;

    }

    return true;

}

/**

 * Loads an ASMap file from disk and validates it.

 */

std::vector<std::byte> DecodeAsmap(fs::path path)

{

    FILE *filestr = fsbridge::fopen(path, "rb");

    AutoFile file{filestr};

    if (file.IsNull()) {

        LogWarning("Failed to open asmap file from disk");

        return {};

    }

    int64_t length{file.size()};

    LogInfo("Opened asmap file %s (%d bytes) from disk", fs::quoted(fs::PathToString(path)), length);

    // Read entire file into memory

    std::vector<std::byte> buffer(length);

    file.read(buffer);

    if (!CheckStandardAsmap(buffer)) {

        LogWarning("Sanity check of asmap file %s failed", fs::quoted(fs::PathToString(path)));

        return {};

    }

    return buffer;

}

/**

 * Computes SHA256 hash of ASMap data for versioning and consistency checks.

 */

uint256 AsmapVersion(const std::span<const std::byte> data)

{

    if (data.empty()) return {};

    HashWriter asmap_hasher;

    asmap_hasher << data;

    return asmap_hasher.GetHash();

}