nx_av1_ec.nx

nx_av1_ec.nx -- the AV1/AV2 multi-symbol CDF arithmetic decoder. THE MOAT FOUNDATION. AV1, AVIF and AV2 all rest on this one primitive: a multi-symbol (not binary) arithmetic coder whose probability tables ADAPT as they decode. Nothing in this tree had it. nx_rangecoder.nx is the older BINARY range coder (CABAC/VP8 class) -- it cannot decode AV1, because AV1 draws an N-ary symbol per call against a cumulative distribution rather than a sequence of bits against a single probability. AV2 v1.0.0 (spec frozen 2026-05-28) inherits this coder essentially intact, so this module is the shared substrate for both. Build it once, correctly. CDF CONVENTION -- FORWARD, PER THE SPEC. cdf[i] is the cumulative probability of symbols <= i scaled to 32768, so the array INCREASES and cdf[N-1] == 32768. libaom stores the INVERSE internally (32768 minus this, decreasing to 0) and its update formula is written for that form. Mixing the two is the classic porting error and it is nearly invisible: the terminating entry agrees under BOTH conventions, so the first symbol of a stream often decodes correctly and everything after it diverges. This file is spec-form throughout; do not paste libaom arithmetic into it. THE TRAILING SLOT. cdf has N+1 entries: N probabilities plus a COUNTER at cdf[N] that tracks how many times this context has been used. The counter drives the adaptation rate -- a fresh context adapts fast, a mature one slowly. Dropping it (sizing the array at N) both breaks adaptation and writes one past the end. genealogy_id: av1_spec_8_3_symbol_decoder lineage_id: nx_av1_ec_v1 license_tier: ORIGINAL