price-watch

A small Zig project for learning through a practical price checker. It reads products from pages.config, fetches each page, extracts a price between configured HTML markers, and prints the result in minor units.

Note

As this project is for learning the language / syntax. I decided to do a lot of string manipulations manually instead of using std.mem

Status

The project currently supports:

• Reading product names and URL extraction rules from pages.config.
• Fetching HTTP pages and extracting text between start and end markers.
• Removing a leading $ and commas before converting a price to minor units.
• Configuring the currency and whether a value is already in minor units.

Price monitoring, persistence, and reporting price changes are still to be built.

Project Layout

• src/main.zig: executable entry point. Reads the config and extracts every product URL.
• src/file_reader.zig: parses pages.config into Product and ProductUrl values.
• src/html.zig: fetches a URL and returns its response body.
• src/extract.zig: defines products and extracts, formats, and converts prices.
• pages.config: product and extraction-rule configuration.
• product-sites/: React Router practice storefronts for local scraper testing.

Configuration

Each product starts with a bracketed name. Add one or more URL rules below it:

[Product name]
URL | start marker | end marker | currency | is_minor

currency and is_minor are optional:

URL | start marker | end marker
URL | start marker | end marker | USD
URL | start marker | end marker | USD | true

The defaults are AUD and false. is_minor accepts only true or false. Whitespace around fields is ignored.

For example, this extracts the value of data-price from a local product page:

[VOLTAGE 9 Neural Deck]
http://127.0.0.1:5173/product-1 | data-price=" | " | USD | false

When is_minor is true, the parsed value is already a minor-unit amount and is not multiplied by 100.

Running

Build the executable:

zig build

Run it:

./zig-out/bin/price-watch

Local Test Sites

Start the React Router practice sites from product-sites/ in a separate terminal:

bun run dev -- --host 127.0.0.1

The explicit IPv4 host is required so the Zig HTTP client can reach the site at http://127.0.0.1:5173.

• /product-1: formatted data-price value.
• /product-2: data-quote-price value.
• /product-3: cents stored in data-price; configure it with is_minor set to true.
• /product-4: formatted data-current-price value.

Verify the React app from product-sites/ with:

bun run typecheck
bun run build

Testing

zig test src/file_reader.zig
zig test src/extract.zig

Memory Ownership

parse_file copies product names, URLs, markers, and configured currencies with the allocator it receives. In main.zig, those copies are owned by the arena and remain valid until arena.deinit().

html.get_page_content also allocates response bodies with the passed allocator. The current program uses the same arena for all work, releasing parser data, URL lists, and response bodies together at shutdown.

Current Limitations

• Products and price history are not persisted.
• Price extraction depends on retailer-specific HTML markers, which can change when a page is redesigned.
• Each configuration line must fit in the parser's 1024-byte read buffer and end with a newline.