doc: move trees-algo to tech-spec

Author: zargot

docs/spec/tech-spec.md

@@ -365,6 +365,57 @@ Example rules with its generated tree:
             (select, "all")
 ```
 
+#### Algorithm
+
+- Each rule in a sharing CSV sheet can only reference rules defined in a
+  previous row.
+- a node has a type/kind, a value, and a list of children
+- the children of a node is called 'sons' since it's shorter
+- nodes are first added to ctx and then later added to parent nodes with O(1)
+  lookup, this way the tree is constructed incrementally while parsing each
+  rule
+- share nodes are made to be children of a single root-node, since each org
+  gets its own node, there may be multiple share-rules, and the tree
+  can only have a single root node
+- the root-node is updated every time a new share-node is added
+- tables of each rule are cached for O(1) lookup
+
+```
+for each rule:
+    for each table in rule, or just once if no table:
+        init node, depending on rule mode:
+            select:
+                kind = select
+                value = empty if sons are specified, otherwise 'all'
+                sons = a value node for each column name
+            filter:
+                kind = filter
+                value = operator
+                sons =
+                    1. a key node for the field name
+                    2. a value node for each filter value
+            group:
+                kind = group
+                value = operator
+                sons = nodes matching the rule's list of ids
+            share:
+                kind = root
+                sons =
+                    for each organization:
+                        kind = share
+                        value = org
+                        sons =
+                            for each select-node referenced in share-rule:
+                                for each table in select-node's rule:
+                                    kind = "table"
+                                    value = table
+                                    sons =
+                                        1. select node
+                                        2. filter/group node referenced in
+                                           share-node. Multiple nodes are
+                                           implicitly grouped with an AND-node.
+```
+
 ### SQL query generation
 
 SQL queries are (recursively) generated from each table-node of the AST. Values