core/clock/tide.c
| Line | Branch | Exec | Source |
|---|---|---|---|
| 1 | /** | ||
| 2 | * @file tide.c | ||
| 3 | * @brief The rise and fall of a semidiurnal tide, from a plain running minute count. | ||
| 4 | * | ||
| 5 | * @ingroup lib_core | ||
| 6 | */ | ||
| 7 | #include "clock/tide.h" | ||
| 8 | |||
| 9 | /// The cycle's length cubed, which the smoothstep divides by | ||
| 10 | #define TIDE_PERIOD_CUBE ((int64_t)TIDE_PERIOD_MIN * TIDE_PERIOD_MIN * TIDE_PERIOD_MIN) | ||
| 11 | |||
| 12 | /** | ||
| 13 | * @brief Where in the cycle a minute count lands. | ||
| 14 | * | ||
| 15 | * C truncates a negative division towards zero, which leaves a negative remainder, so this | ||
| 16 | * pulls it back up by hand. | ||
| 17 | * | ||
| 18 | * @param minutes A running minute count, on the same base as tide_level. | ||
| 19 | * @return The phase within the cycle, 0 to TIDE_PERIOD_MIN - 1. | ||
| 20 | */ | ||
| 21 | 2998 | static int32_t phase(int32_t minutes) | |
| 22 | { | ||
| 23 | 2998 | int32_t within = minutes % TIDE_PERIOD_MIN; | |
| 24 | |||
| 25 |
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2998 | return within < 0 ? within + TIDE_PERIOD_MIN : within; |
| 26 | } | ||
| 27 | |||
| 28 | 2996 | int tide_level(int32_t minutes) | |
| 29 | { | ||
| 30 | // twice the phase, folded back at the turn, so it climbs from 0 to the period over the first | ||
| 31 | // half of the cycle and comes back down over the second | ||
| 32 | 2996 | int32_t twice = phase(minutes) * 2; | |
| 33 |
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2996 | int32_t climb = twice < TIDE_PERIOD_MIN ? twice : 2 * TIDE_PERIOD_MIN - twice; |
| 34 | |||
| 35 | // the smoothstep curve 3t^2-2t^3 of climb over the period, scaled to 0..100. it flattens both | ||
| 36 | // turns and steepens the middle | ||
| 37 | // it is worked out in one go with one rounding at the end, so the level moves at most one point | ||
| 38 | // a minute and low and high water last about as long as each other | ||
| 39 | // the product on top runs past 40 billion, so it is worked in 64 bits | ||
| 40 | 2996 | int64_t top = (int64_t)100 * climb * climb * (3 * TIDE_PERIOD_MIN - 2 * climb); | |
| 41 | |||
| 42 | 2996 | return (int)((top + TIDE_PERIOD_CUBE / 2) / TIDE_PERIOD_CUBE); | |
| 43 | } | ||
| 44 | |||
| 45 | 2 | bool tide_rising(int32_t minutes) | |
| 46 | { | ||
| 47 | 2 | return phase(minutes) * 2 < TIDE_PERIOD_MIN; | |
| 48 | } | ||
| 49 |