I've finally managed something that doesn't depend on a lookup table of one form or another and is somewhat neat.
Dual Fader - two fader circuits, the top one fading at 2 levels per (2rt) period, the bottom one fading by one but only while the top one still has power.
There's a reset that clears any residual power before the new input is latched, and there's output enable feature that keeps output unpowered while the calculation is in progress.
Due to timing dependencies it has a small, rather harmless quirk - maps 0->0, 1->1, 14->8, 15->9. Making the input signal 1 weaker (just adding one redstone dust on input)
Following: top view of the entire device, and top view onto the bottom fader (top fader removed).
The right side of the image is a monostable circuit that activates on input change and passes input to the divider, then cuts it off to enable calculation. It also activates the two reset lines (above). The central part - two comparators aimed into solid blocks - create analog memory cells. Combined with comparators (bottom sides of the images) subtracting signal strength 2 or 1 respectively (produced by reading the appropriately filled composters) these comprise the fader circuits.
The composter of the lower circuit is lowered into position by a sticky piston activated by signal from the upper circuit.
The left side has a piston raising a solid block to disable output while calculation is in progress and lowering it between two comparators to enable output.