Remove the subtree edge case entirely by allowing zero-length subtrees - #275
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The accumulated test for the subtree covering was written as if the single-subtree special case was for [start, end) being a subtree, when it's actually for [start, end) having size 1. Either would have worked. The spec mostly arbitrarily picked the latter.
Instead of having an rare edge case where landmark intervals of size one return one subtree instead of two, just always return two. In order to do that, we have to say zero-length subtrees are legal. That introduces silly special cases for consistency proofs, but means all our functions have uniform types, which I think is worth it.
davidben
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Jul 18, 2026
| ## Subtree Inclusion Proofs {#subtree-inclusion-proof-vectors} | ||
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| For each value of `end` from 1 to 130, and each value of `start` from 0 to `end - 1`, if `[start, end)` is a valid subtree, for each value of `index` from `start` to `end - 1`, add to the rolling hash the ASCII string `INDEX [START, END)`, then, for each hash in the inclusion proof ({{subtree-inclusion-proofs}}) for `d[index]` in the subtree `[start, end)`, a space (U+0020) followed by the hexadecimal encoding of that hash, and finally a newline (U+000A), where `INDEX` is the decimal representation of `index`, and `START` and `END` are the decimal representations of `start` and `end`, respectively. | ||
| For each value of `end` from 0 to 130, and each value of `start` from 0 to `end`, if `[start, end)` is a valid subtree, for each value of `index` from `start` to `end - 1`, add to the rolling hash the ASCII string `INDEX [START, END)`, then, for each hash in the inclusion proof ({{subtree-inclusion-proofs}}) for `d[index]` in the subtree `[start, end)`, a space (U+0020) followed by the hexadecimal encoding of that hash, and finally a newline (U+000A), where `INDEX` is the decimal representation of `index`, and `START` and `END` are the decimal representations of `start` and `end`, respectively. |
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The hash is unchanged because all the extra loop iterations are are vacuous. I added them just to keep it all uniform.
lukevalenta
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Jul 20, 2026
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Oh this is a really nice simplification. LGTM.
Co-authored-by: Luke Valenta <lvalenta@cloudflare.com>
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This includes #274
Instead of having an rare edge case where landmark intervals of size one return one subtree instead of two, just always return two. In order to do that, we have to say zero-length subtrees are legal.
That introduces silly special cases for consistency proofs, but means all our functions have uniform types, which I think is worth it.