How Solana Programs Plug Into Each Other (Composability)

use anchor_lang::prelude::*;
use anchor_spl::token::{self, Token, TokenAccount, Transfer};

#[derive(Accounts)]
pub struct Swap<'info> {
    #[account(mut)]
    pub user_source: Account<'info, TokenAccount>,
    #[account(mut)]
    pub user_dest: Account<'info, TokenAccount>,
    #[account(mut)]
    pub pool_source: Account<'info, TokenAccount>,
    #[account(mut)]
    pub pool_dest: Account<'info, TokenAccount>,
    pub pool_authority: Signer<'info>,
    pub token_program: Program<'info, Token>,
}

pub fn swap(ctx: Context<Swap>, amount_in: u64, min_out: u64) -> Result<()> {
    // Transfer user -> pool (user signs)
    let cpi_in = CpiContext::new(
        ctx.accounts.token_program.to_account_info(),
        Transfer {
            from: ctx.accounts.user_source.to_account_info(),
            to: ctx.accounts.pool_source.to_account_info(),
            authority: ctx.accounts.user_source.to_account_info(),
        },
    );
    token::transfer(cpi_in, amount_in)?;

    // Compute amount_out using x*y=k pricing on pool reserves
    let reserve_in = ctx.accounts.pool_source.amount;
    let reserve_out = ctx.accounts.pool_dest.amount;
    let amount_out = (reserve_out * amount_in) / (reserve_in + amount_in);
    require!(amount_out >= min_out, SwapError::Slippage);

    // Transfer pool -> user (program-derived authority signs)
    let cpi_out = CpiContext::new_with_signer(
        ctx.accounts.token_program.to_account_info(),
        Transfer {
            from: ctx.accounts.pool_dest.to_account_info(),
            to: ctx.accounts.user_dest.to_account_info(),
            authority: ctx.accounts.pool_authority.to_account_info(),
        },
        &[],
    );
    token::transfer(cpi_out, amount_out)?;
    Ok(())
}

#[error_code]
pub enum SwapError { Slippage }