[17krishna8]

Description

astcenc_context_alloc() accepts a hand-built astcenc_config in addition to one populated by astcenc_config_init(). validate_block_size() treats a block_z of zero as meaning "2D", the same as a value of one, and astcenc_config_init() normalizes its own block_z input to at least one before storing it (astcenc_entry.cpp line 527). However validate_config() did not apply the same normalization to the copy of the config stored in the context, so a context can be created holding block_z == 0.

The stored block dimensions are later used as divisors when computing block counts:

// astcenc_compress_image() / astcenc_decompress_image()
size_t blocks_z = astc::get_block_count_safe(dim_z, block_z);

A config with block_z set to zero therefore causes a division by zero (SIGFPE) in get_block_count_safe() on the first encode or decode call, instead of behaving like the equivalent 2D config with block_z == 1.

Reproducer

astcenc_config config;
astcenc_config_init(ASTCENC_PRF_LDR, 6, 6, 1, ASTCENC_PRE_FASTEST, 0, &config);
config.block_z = 0;                       // treat zero as "2D"

astcenc_context* ctx;
astcenc_context_alloc(&config, 1, &ctx, nullptr);   // accepted

astcenc_decompress_image(ctx, ...);        // SIGFPE: dim_z / block_z

Changes

• Normalize the stored block_z to at least one in validate_config(), which runs on every astcenc_context_alloc() call, matching the normalization astcenc_config_init() already applies to its own inputs.
• Add a unit test covering a config with block_z == 0 (fails with a divide-by-zero or a wrong context config on the old code, passes after the fix).

Behavior is otherwise unchanged for any config built through astcenc_config_init(), which already normalizes block_z to at least one.