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The last codec ADR 0015 deferred. The plan recorded the blocker as a dependency decision, not an algorithm: openjpeg would add a C dependency that breaks the Android cross-compile. This is pure Rust and adds no dependency at all. It shares the MQ arithmetic decoder with JBIG2 — T.800 and T.88 specify the same coder — so the previous tranche paid for most of this one. Context::with_state moved onto the shared type because JPEG 2000 starts three of its nineteen contexts away from state 0 and JBIG2 starts all of them at 0. Implemented: codestream and JP2 container parsing, packet headers with tag trees and the bit-stuffing rule, EBCOT tier-1 (all three passes, four zero-coding context tables, run-length mode), both 5/3 reversible and 9/7 irreversible wavelets, RCT and ICT, arbitrary decomposition levels, and multiple components. Refused by name: multiple tiles, custom precinct partitions, code-block style options, COC/QCC/RGN/POC overrides, subsampled components. Each error says which feature the file needs. This matters more here than anywhere else in the stack, because a JPEG 2000 decoder that quietly skips something does not fail — it returns a slightly soft or banded image that looks entirely fine. That property also dictates how this is tested. Fixtures are produced by OpenJPEG via Pillow and compared **exactly**, sample for sample: the fixtures are lossless 5/3 so no tolerance is needed, and a tolerance is where a subtly wrong decoder hides. Four images — grayscale raw codestream, the same in a JP2 container, a larger one whose tag trees actually branch, and RGB. A generator script is checked in beside them so CI can prove the fixtures still match what produced them. Verified by mutation. The first round was misleading and is worth recording, because it is the same lesson as ADR 0017: DC level shift dropped 3 fail 5/3 lifting rounding changed 2 fail RCT sign flipped PASSED <- survived RCT components swapped PASSED <- survived cleanup run-length disabled PASSED <- survived sign-context XOR dropped PASSED <- survived Four mutations survived because Pillow writes MCT=0 by default, so the RGB fixture coded its three components independently and never reached the colour transform at all. The RCT branch was completely untested while appearing covered — an untested branch that looks tested is worse than one that looks missing. Added rgb8_mct.j2k with mct=1; all four now fail. The header bit-stuffing mutation is caught by the unit test rather than the round-trip. Two real defects found while writing the tests: - A corrupt marker length in a tile-part header walked the read cursor past the codestream and panicked on a slice. Found by the corruption sweep, not by review. The sweep now truncates at every length and flips every byte of a real file, and asserts only that nothing panics. - The 9/7 flat-signal test initially asserted an amplitude I had derived from my own arithmetic. That is a test agreeing with the code by construction. It now asserts flatness — a ripple means the lifting or the edge extension is wrong — and the amplitude is pinned by the OpenJPEG round-trips instead, which use pixels this code did not produce. Also removed two dead fields and an unused parameter that clippy found: Subband::x0/y0 are always zero in the single-tile case this supports, and dead state implying multi-tile support exists is worse than no state. JPX decodes on the image path, like JBIG2, because the codestream carries its own geometry; it stays in REFUSED_CODECS with a reason string saying where it is decoded rather than that it is missing. Engine suite 866 -> 920. Coverage 85.66% -> 86.16%; jpx.rs at 93.72% with a floor at 88. Phase 3 is complete: CCITT, JBIG2 and JPX all land, and the plan is updated to say so and to record how the two gating questions — JBIG2's CVE record and JPX's C dependency — were actually answered. |
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