median v0.1.0

Return the median of the last N samples.

Application scenarios

• PIR or ADC samples contain isolated impulses.
• A key or limit switch produces contact bounce.
• A battery reading is disturbed by a load pulse.

Contract

void median_filter_init(median_filter_t *filter, uint16_t *window, uint8_t size);
uint16_t median_filter_step(median_filter_t *filter, uint16_t sample);
void median_filter_reset(median_filter_t *filter);

• The caller supplies window, an array of size elements.
• size must be 1..255. Odd sizes give the true median; even sizes return the lower middle value.
• No scratch buffer and no allocation.
• Cost is O(N^2) per sample; use small N (3, 5, or 7).

Parameters

• Odd size; a larger window removes more impulses but adds delay of about (size-1)/2 samples.
• window must remain valid for the filter lifetime.

Memory and timing

• State is 4 bytes plus the caller's 2*size bytes.
• O(N^2) comparisons; at N=7 this is 49 comparisons.

Integration

uint16_t window[5];
median_filter_t filter;
median_filter_init(&filter, window, 5);
uint16_t clean = median_filter_step(&filter, adc_read());

When not to use

• A fast control loop with a large window; the delay and O(N^2) cost are too high.
• A stream with no impulses; moving-average is cheaper.

Verification

make test covers odd/even windows, wrap, reset, size 1, and size 0.

Behavior reference: a production median filter; this implementation is original.

uint16_t *window;
uint8_t size;
uint8_t count;
uint8_t index;