1819893 - Add negate node to calc tree to replace the mul_by in sum nodes

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, enhancement)

Description

[Tiaan Louw [:tlouw]]

Assumption in calc(): x - y == x + (-1y))

This used to work, because multiply and divide was always evaluated at parse time before add and subtract, but the plan is to add a product node so that evaluation happens at "simplify/evaluate" time. Adding the product node avoids us from violating distributivity in newer nodes like sign(). (see bug 1815448)

Example in old code:

// multiply happens at parse time
calc(40px + 10% - 20% / 2) == calc(40px + 10% - 10%)

// multiply now creates a new node
calc(40px + 10% - 20% / 2) == calc(40px + 10% - (20% / 2)) != calc(40px + 10% - 20% / -2)

To fix this we can't *-1 for subtractions any more, so each node in a +/- should keep track of it's own operation. This makes storing the terms in a list very ugly, so the plan is to convert the node into a single operation node. E.g.

Sum(vec<Node>)

changes to:

Sum { left: Node, right: Node, op: AddOrSubtract }

Initially this will cause some additional allocations and deeper stack, but we should be able to inline some operations if they have the same unit/type, etc.

Comment 1

[Tiaan Louw [:tlouw]]

Original plan to create binary sum nodes is not needed as adding negate nodes as suggested by the spec works as well.

Comment 2

[Tiaan Louw [:tlouw]]

Attachment: Bug 1819893 - Add negate node to use in place of mul_by in sum nodes r=emilio

Sum nodes would use mul_by to negate nodes to do subtraction, but some
nodes are not distributive. This patch adds a negate node, so that the
operations inside these negate nodes can be resolved first and then the
"subtraction" can be applied.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/9dc528bebd61
Add negate node to use in place of mul_by in sum nodes r=emilio

Comment 4

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/9dc528bebd61