doc: Add more extensive docs to asmap implementation

diff --git a/contrib/asmap/asmap.py b/contrib/asmap/asmap.py
index 2ae84a3f..292048b6 100644
--- a/contrib/asmap/asmap.py
+++ b/contrib/asmap/asmap.py
@@ -157,7 +157,7 @@ class _Instruction(Enum):
     JUMP = 1
     # A match instruction, encoded as [1,1,0] inspects 1 or more of the next unused bits
     # in the input with its argument. If they all match, execution continues. If they do
-    # not, failure is returned. If a default instruction has been executed before, instead
+    # not, failure (represented by 0) is returned. If a default instruction has been executed before, instead
     # of failure the default instruction's argument is returned. It is followed by an
     # integer in match encoding, and a subprogram. That value is at least 2 bits and at
     # most 9 bits. An n-bit value signifies matching (n-1) bits in the input with the lower
diff --git a/src/util/asmap.cpp b/src/util/asmap.cpp
index a40a0615..ed1d3b0b 100644
--- a/src/util/asmap.cpp
+++ b/src/util/asmap.cpp
@@ -21,10 +21,36 @@
 #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;
@@ -32,6 +58,10 @@ inline bool ConsumeBitLE(size_t& bitpos, std::span<const std::byte> bytes) noexc
     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;
@@ -39,24 +69,43 @@ inline bool ConsumeBitBE(uint8_t& bitpos, std::span<const std::byte> bytes) noex
     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;
+    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) {
-        if (bit_sizes_it + 1 != bit_sizes.end()) {
+        // 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;
+            bit = 0;  // Last class has no continuation bit
         }
         if (bit) {
-            val += (1 << *bit_sizes_it);
+            // 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);
+                val += bit << (*bit_sizes_it - 1 - b); // Big-endian within the class
             }
             return val;
         }
@@ -64,40 +113,72 @@ uint32_t DecodeBits(size_t& bitpos, const std::span<const std::byte> data, uint8
     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};
@@ -110,38 +191,53 @@ uint32_t Interpret(const std::span<const std::byte> asmap, const std::span<const
     while (pos < endpos) {
         opcode = DecodeType(pos, asmap);
         if (opcode == Instruction::RETURN) {
+            // Found leaf node - return the ASN
             default_asn = DecodeASN(pos, asmap);
             if (default_asn == INVALID) break; // ASN straddles EOF
             return default_asn;
         } else if (opcode == Instruction::JUMP) {
+            // Binary branch: if IP bit is 1, jump forward; else continue
             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)) {
-                pos += jump;
+            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
             match = DecodeMatch(pos, asmap);
             if (match == INVALID) break; // Match bits straddle EOF
-            matchlen = std::bit_width(match) - 1;
+            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 (uint32_t bit = 0; bit < matchlen; bit++) {
                 if (ConsumeBitBE(ip_bit, ip) != ((match >> (matchlen - 1 - bit)) & 1)) {
-                    return default_asn;
+                    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
         }
     }
-    assert(false); // Reached EOF without RETURN, or aborted (see any of the breaks above) - should have been caught by SanityCheckASMap below
+    // 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};
@@ -149,12 +245,16 @@ bool SanityCheckASMap(const std::span<const std::byte> asmap, int bits)
     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;
+    bool had_incomplete_match = false;  // Track <8 bit matches for efficiency check
+
     while (pos != endpos) {
-        if (!jumps.empty() && pos >= jumps.back().first) return false; // There was a jump into the middle of the previous instruction
+        // 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) {
-            if (prevopcode == Instruction::DEFAULT) return false; // There should not be any RETURN immediately after a DEFAULT (could be combined into just 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()) {
@@ -179,20 +279,22 @@ bool SanityCheckASMap(const std::span<const std::byte> asmap, int bits)
             --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);
+            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;
-            if (matchlen < 8 && had_incomplete_match) return false; // Within a sequence of matches only at most one should be incomplete
+            // 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) {
-            if (prevopcode == Instruction::DEFAULT) return false; // There should not be two successive DEFAULTs (they could be combined into one)
+            // 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;
@@ -203,6 +305,10 @@ bool SanityCheckASMap(const std::span<const std::byte> asmap, int bits)
     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)) {
@@ -212,6 +318,9 @@ bool CheckStandardAsmap(const std::span<const std::byte> data)
     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");
@@ -223,6 +332,7 @@ std::vector<std::byte> DecodeAsmap(fs::path path)
     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);
 
@@ -234,6 +344,9 @@ std::vector<std::byte> DecodeAsmap(fs::path path)
     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 {};