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Mojo function

dequant_fp4_nibble

def dequant_fp4_nibble[out_type: DType](packed_byte: UInt8, nibble_hi: Bool, scale_abs: Float32) -> Scalar[out_type]

Dequantizes one E2M1 nibble of a packed byte to out_type.

Low nibble (nibble_hi=False) is the even K element; high nibble is the odd one. The 4-bit value indexes E2M1_TO_FLOAT32 (sign + magnitude already baked into the 16-entry LUT) and is scaled by the block's |scale|.

Parameters:

out_type (DType): Output element dtype (bf16 for the Apple W4A16 path).

Args:

packed_byte (UInt8): The two-nibble packed FP4 byte.
nibble_hi (Bool): True selects the high nibble (byte >> 4), False the low.
scale_abs (Float32): The block scale, already passed through abs().

Returns:

Scalar[out_type]: The dequantized value E2M1_TO_FLOAT32[nibble] * scale_abs as Scalar[out_type].