= Prototype Implementation =

The prototype is a Go command-line application in
[https://github.com/StefanTrsunov/bp/tree/main/server server/] that works against the `project`
schema in PostgreSQL. It implements all seven use cases from
[https://github.com/StefanTrsunov/bp/blob/main/docs/P3-UseCaseModel/UseCaseModel.md UseCaseModel]
– the rubric requires at least three – with every database access shown as real, executed SQL.
An auxiliary program in [https://github.com/StefanTrsunov/bp/tree/main/bots bots/] simulates a
live market so prices move while the prototype is running.

Build, configure, run and test instructions:
[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/BuildInstructions.md BuildInstructions].

All pages listed below, together with the screenshots of each run, are kept in the project's
GitHub repository, [https://github.com/StefanTrsunov/bp StefanTrsunov/bp], under
`docs/P4-Prototype/`.

== Implemented use-cases ==

||=Page=||=Use-case=||=Source=||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0001Implementation.md UseCase0001Implementation]||Register a new account||[https://github.com/StefanTrsunov/bp/blob/main/server/auth.go server/auth.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0002Implementation.md UseCase0002Implementation]||Log in||[https://github.com/StefanTrsunov/bp/blob/main/server/auth.go server/auth.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0003Implementation.md UseCase0003Implementation]||Deposit virtual funds||[https://github.com/StefanTrsunov/bp/blob/main/server/account.go server/account.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0004Implementation.md UseCase0004Implementation]||Place market BUY order||[https://github.com/StefanTrsunov/bp/blob/main/server/trade.go server/trade.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0005Implementation.md UseCase0005Implementation]||Place market SELL order||[https://github.com/StefanTrsunov/bp/blob/main/server/trade.go server/trade.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0006Implementation.md UseCase0006Implementation]||View portfolio and history||[https://github.com/StefanTrsunov/bp/blob/main/server/portfolio.go server/portfolio.go]||
||[https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/UseCase0007Implementation.md UseCase0007Implementation]||Manage watchlist||[https://github.com/StefanTrsunov/bp/blob/main/server/watchlist.go server/watchlist.go]||

Each page mirrors its P3 use-case page and adds the actual SQL emitted by the Go code plus a
screenshot of the corresponding run against the live database. The screenshots are committed
alongside the pages, in
[https://github.com/StefanTrsunov/bp/tree/main/docs/P4-Prototype/screenshots docs/P4-Prototype/screenshots/].

== What the prototype demonstrates about the database design ==

 * '''The current price is never stored as a column.''' It is always the price of the most recent
   row in `market_trades`, read through the `v_latest_prices` view. Both the user's own fills and
   the bot's simulated trades feed the same table, so there is exactly one definition of "the
   price".
 * '''Money movements are transactional.''' Buying touches five tables – `orders`, `users`,
   `holdings`, `transactions`, `market_trades` – inside one transaction. A failed balance check
   rolls the whole thing back: after a rejected purchase there is no order row, no ledger entry
   and no holding. This is verified in the failure-path tests in
   [https://github.com/StefanTrsunov/bp/blob/main/docs/P4-Prototype/BuildInstructions.md BuildInstructions].
 * '''Constraints do real work.''' `UNIQUE (user_id, crypto_id)` on `holdings` is what makes the
   `INSERT … ON CONFLICT DO UPDATE` upsert possible, so the weighted-average entry price is
   recomputed by the database in one statement instead of by a read-modify-write in application
   code.
 * '''No identifiers are ever typed.''' Markets are listed with their prices before any choice is
   made, and everything else is selected by symbol.

== Known limitations ==

Deliberately out of scope for a first prototype, and the natural content of the later phases:

 * Only `market` orders execute. `limit` is accepted by the schema (`orders.type`) but the
   matching logic is not implemented.
 * Passwords are SHA-256 without a salt. Adequate to demonstrate that the password itself is
   never stored; not adequate for real use. A proper password hash belongs in P9 (security).
 * Money is handled as `float64` in Go while the database columns are `numeric`. All arithmetic
   that must be exact – the weighted average – is done in SQL for that reason, but the Go side
   would need a decimal type for real use.
 * There is no connection pooling configuration and no explicit isolation level; both are P8
   topics.