token-bonding-curve is forked from anchor-token-swap (which is a fork of spl token-swap with anchor added) with a custom LinearPriceCurve type added in src/curve/linear_price.rs. It models a curve where the price of the output token b increases at a linear rate as more collateral token a has been swapped in. See docs in linear_price.rs for more calculation details and cavets.
e.g. a curve with formula a = 3b + 2 – where a is the price of a single bonded token b (denominated in amount of token a) and b is the amount of token b that's been swapped out of this curve – starts at a price of 2 token A in required to get 1 token B out when 0 token b has been exchanged and increases by 3 token A to get 1 token B out for every 1 token b that's swapped out
Under the hood it uses the integral of the price formula to calculate the amount of token a locked in the curve and uses that to determine the spot price and the amount of destination token to emit
Pool tokens and deposits/withdrawals of pool tokens are intentionally disabled so that liquidity can't be added/removed from the swap outside of the swap instruction. If more liquidity is required, a second curve can be initialized with the same slope and an appropriately set start price (e.g. the end price of the previous curve). Fees are also disabled (at the instruction level, see lib.rs:initialize_linear_price).
See https://github.com/rally-dfs/anchor-token-swap/blob/main/README.md and https://github.com/solana-labs/solana-program-library/tree/master/token-swap where this was forked from too
The main tests (that weren't already in spl token) are in linear_price.rs and dfs_precise_number.rs
$ cargo test --package token-bonding-curve --lib -- dfs_precise_number::tests linear_price::tests
and in token-bonding-curve.ts. This takes a lot longer to run than the rs tests since it's actually making end to end calls to the validator, but it's the only way to test that we aren't overflowing compute.
$ ANCHOR_WALLET=~/.config/solana/id.json anchor test
(Note thisrequires installing anchor CLI via cargo install, e.g. cargo install --git https://github.com/project-serum/anchor --tag v0.20.1 anchor-cli --locked)