Bitcoin halving
Fifty coins, then half of that, on a schedule written into the client.
Satoshi Nakamoto and Bitcoin Core. GetBlockSubsidy.
The Bitcoin client pays the miner who finds a block. At the beginning that payment is a subsidy of 50 BTC, a reward the software creates rather than a fee a user attaches. Fees came later, as a second stream, when blocks filled and people bid for room. The subsidy was the plan for issuing the supply. It does not stay at 50. Every 210,000 blocks the reward is cut in half. The comment in the current source says the interval is about four years, because the chain aims at a block every ten minutes and 210,000 of those intervals land near a presidential term, or an Olympiad, depending on the metaphor you prefer. The code does not contain the metaphor. It contains a division and a shift.
GetBlockSubsidy is that policy in one short function. It divides the height of the chain by the interval. The quotient is how many times the reward has already been halved. A shift of 64 or more is refused, because a 64-bit word shifted that far is not a useful coinbase. The function starts from 50 times the base unit, the smallest unit being a hundred-millionth of a coin, and shifts right by the count of halvings. Right shift on an integer is a halving that drops the fraction. There is no floating point in the monetary base. When the shift has eaten the integer, the subsidy is zero and only fees remain.
The arithmetic is also a sum. A geometric series of halves, 50 plus 25 plus 12.5 and so on, taken across intervals of 210,000 blocks, converges on 21 million coins. The cap is not a separate decree stored in a table of yearly quotas. It is what you get if you add up the function. That is why a dozen lines can be a monetary policy. Change the interval or the initial subsidy and the sum changes. Leave them alone and the supply is spoken for.
Satoshi Nakamoto wrote the first client and the white paper that argued for the chain. The genesis block, in January 2009, carried a headline from a newspaper, a timestamp that the chain did not begin in secret years earlier. The subsidy rule was already in the client. Nakamoto stopped writing in public. Bitcoin Core, the descendant of that client, maintains the function now. Both copyright lines appear in the MIT notice on the product page: the developers, and the name the project uses for the longer authorship. The notice is the condition of the license. It has to travel with substantial copies of the software. A mug is a poor place for a license, so the notice stays here, and the bars are the function alone.
The first cut arrived in 2012, when the height crossed 210,000 and the subsidy became 25. Later crossings, in 2016, 2020, and 2024, cut it again, to 12.5, then 6.25, then 3.125. Each date was a crowd watching an integer. The function does not know it is being watched. It knows a height. Miners who had built businesses on 50 coins per block learned, on a schedule they could have read in the source, that the schedule was the product. Markets priced the cut before the block arrived. The code still did the same shift.
The bars are the whole function, and they are short on purpose. A long function would have looked like ordinary software. This one looks like a rule. Color marks the constant, the shift, the guard against a ridiculous count. There is no chart of price and no portrait of a founder. The founder is a pseudonym, and the price is not in the file. What is in the file is the issuance. The MIT permission text, including the requirement that the notice ride along with the software, is on the product page in full. The ceramic gets the shift.
Nothing in the function reaches outside the process. It does not ask a server what the reward should be, and it does not read a policy file an operator might edit. Two implementations that share the height compute the same subsidy or they are not on the same chain. That is the point of writing a monetary rule as a pure function of height. The bars are short because the rule is short. The notice on the product page is long because the license, unlike the subsidy, still needs its paragraph.