http://tumblr.g.hatena.ne.jp/saitamanodoruji/20130725/1374700510 のC版

EndlessSummer.c

//
//  EndlessSummer.c
//  Tumbltail
//
//  Created by nya on 2013/11/05.
//
//

#import "EndlessSummer.h"


typedef struct {
	double a1;
	int a2;
	double b1;
	int b2;
	long long upperBound;
} ESParamBase;

#define PARAM_COUNT		(10)
#define COEF_A(base)	(base.a1 * pow(10, base.a2))
#define COEF_B(base)	(base.b1 * pow(10, base.b2))
static const ESParamBase baseParams[PARAM_COUNT] = {
	{2.374, -268, 5.342, -10, 1182114958836},
	{2.292, -63, 1.349, -10, 1193135035175},
	{1.214, -11, 3.509, -11, 1241622616806},
	{1.259, -22, 5.546, -11, 1283088065116},
	{3.805, -29, 6.716, -11, 1291075200000},
	{5.579, -28, 6.521, -11, 1292241600000},
	{4.482, -28, 6.541, -11, 1307576237375},
	{2.424, -28, 6.588, -11, 1313586050187},
	{3.923, -13, 3.922, -11, 1340572542928},
	{2.157, -3, 2.249, -11, LLONG_MAX},
};

long long GetPostIDFromUnixTime(long long utime) {
	if (utime < 1149465600000) {
		return -1;
	} else if (utime == LLONG_MAX) {
		return LLONG_MAX;
	}
	for (int i = 0; i < PARAM_COUNT; i++) {
		if (utime <= baseParams[i].upperBound) {
			return ceil(COEF_A(baseParams[i]) * exp(COEF_B(baseParams[i]) * utime));
		}
	}
	return -1;
}

long long GetUnixTimeFromPostID(long long postID) {
	if (postID < 1) { return -1; }
	for (int i = 0; i < PARAM_COUNT; i++) {
		if (postID <= GetPostIDFromUnixTime(baseParams[i].upperBound)) {
			return floor(log(postID / COEF_A(baseParams[i])) / COEF_B(baseParams[i])) / 1000;
		}
	}
	return -1;
}

long long GetRandomPostID() {
	struct tm tm = {
		0, 0, 0, // 秒, 分, 時 (0～)
		1,       // 日 (1～31)
		0,       // 月 (0～11)
		108,     // 年 (1900 年からの年数)
	};
	long long epoch = mktime(&tm);		// 2008/1/1
	long long now = time(NULL) - 60 * 60;
	return GetPostIDFromUnixTime(1000 * (epoch + (now - epoch) * rand() / RAND_MAX));
}