Module decimal 

Encoding for exact decimal columns — the i128 mantissa stream.

A decimal column stores mantissa × 10⁻ˢᶜᵃˡᵉ with the scale held once in the column’s metadata, so what reaches this codec is a column of plain integers. That is the whole argument for the type: the README’s compression section already observes that most float columns are decimals that went through an f64, and that ALP and pco spend their first stage recovering the integer. A decimal column hands that integer over directly — nothing to recover, nothing lost recovering it.

Two physical forms

Almost every real decimal column is narrow. A quarter-hour meter register in watt-hours at four decimal places needs 19 digits to reach 10¹⁴ kWh, so its mantissas fit an i64 with room to spare, and the interesting shapes — a monotone counter, a slowly varying price, a constant tariff — are exactly the shapes the i64 stack is built for.

[form: u8 = 0][inner i64 block: tag + payload]   narrow
[form: u8 = 1][count: u32][first: i128 LE][zigzag varint deltas…]  wide

The narrow form is not a second codec: it delegates to ColumnEncoder::encode_i64, so a decimal column gets RLE on a constant register, frame-of-reference on a narrow range, and pco on everything else — the same trial-and-keep-the- smallest selection an i64 field gets, for free and for ever.

The wide form exists for the columns that genuinely need more than 63 bits of mantissa: 38-digit financial quantities, or a scale so large that ordinary values overflow. Delta + ZigZag + LEB128 keeps a slowly-varying wide column near the narrow one’s size, and a random wide column costs at most 19 bytes per value against the 16 it would take raw.

Structs

DecimalDecoder

Decoder for a column of i128 decimal mantissas.

DecimalEncoder

Encoder for a column of i128 decimal mantissas.