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purebred-mua / hs-notmuch

Modern Haskell binding to the Notmuch mail indexer
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message_get_header: avoid copying header name #12

Closed frasertweedale closed 6 years ago

frasertweedale commented 7 years ago

Observe that message_get_header will be called with a small number of strings (headers), but will be called many times.

Currently, every time message_get_header gets invoked it uses B.useAsCString which allocates (length s + 1) bytes, copies the string (which will be null terminated), and calls the action.

It would be better to define a new type for header names that holds a pointer to a null-terminated string that can be used without further ado. Conversion from B.ByteString would be O(n) but if a distinguished value is used everywhere, the cost need only be incurred once.

A similar approach should be used for tag names, too.

frasertweedale commented 6 years ago

Interning is not worth it (see https://github.com/purebred-mua/hs-notmuch/issues/14). The work to avoid the copying is probably not worth it. Most headers will be read from the parsed mail, not from libnotmuch, to prevent libnotmuch from opening file descriptors to the mail. Therefore, closing this.