; sqlog, a datalog-like system built on sqlite
; may not actually have the full power of datalog OR the full power of sqlite

; datalog has a database of facts, and rules. facts are represented in sqlite directly as tables.
; rules are used by the sqlog engine when generating queries; typically they take the form
; of subqueries used in the WITH RECURSIVE clause.
; It would be possible to store them as views, but the view would need to be regenerated from
; its base rules anytime there is a change, so sqlog needs to know about them no matter what.
; Making sqlog manage them in-memory also allows us to use it to query arbitrary sqlite databases.

; Ideally we would persist rules as JSON in a special table; maybe sqlog_rules?

(local Object (require :core.object))
(local lume (require :lib.lume))

(local Sqlog (Object:extend))

; Generating SQL from Datalog should not be too complex, but it pays to start with the simplest
; case and build up from there.

; simple queries:
; (p x y) -> SELECT p.c1 AS x, p.c2 AS y FROM p
; (p 1 y) -> SELECT p.c2 AS y FROM p WHERE p.c1 = 1
  ; (q x) (p x 1) -> SELECT q.c1 AS x FROM q JOIN p WHERE p.c1 = q.c1 AND p.c2 = 1
; (p 1 2) -> SELECT true FROM p WHERE p.c1 = 1 AND p.c2 = 2
; (p 1 x) (p x 2) -> SELECT t1.c2 AS x FROM p AS t1 JOIN p AS t2 WHERE t1.c1 = 1 AND t1.c2 = t2.c1 AND t2.c2 = 2

; queries using rules:
; [(ancestor x y) (parent x y)] -> SELECT p.c1 AS x, p.c2 AS y FROM parent AS p
; [(ancestor x y) (parent x z) (ancestor z y)] -> SELECT p.c1 AS x, a.y AS y FROM parent AS p JOIN ancestor AS a WHERE p.c1 = a.x AND p.c2 = a.y
; (ancestor x :john) -> WITH RECURSIVE ancestor(x, y) AS (SELECT ... UNION SELECT ...) SELECT a.c1 AS x FROM ancestor AS a WHERE a.y = 'john'

(fn Sqlog.new [self]
  (set self.tables {})
  (set self.rules {}))

(fn Sqlog.deftable [self name ...]
  "Defines the column names of a table and their expected ordering"
  (when (. name self.rules) (error "tables and rules must not overlap"))
  (tset self.tables name [...]))

(fn add-clause [analysis clause] (table.insert analysis.clauses clause))
(fn append-if-missing [list value]
  (when (not (lume.any list #(= $1 value)))
    (table.insert list value)))

(fn Sqlog.reference-name [self analysis name]
  (if (. self.rules name) (append-if-missing analysis.referenced-rules name)
      (. self.tables name) (append-if-missing analysis.tables name)
    (error (.. "Unknown table / rule " name))))

(fn Sqlog.reference-variable [self analysis varname name icolumn]
  (match (. analysis.variable-mapping varname)
    mapping (add-clause analysis [:= mapping [:column name icolumn]])
    nil (do (tset analysis.variable-mapping varname [:column name icolumn])
            (table.insert analysis.variables varname))))

(fn Sqlog.analyze-literal [self analysis literal]
  (match literal
    [:literal name params] (do (self:reference-name analysis name)
                               (each [icolumn value (ipairs params)]
                                 (match value
                                   [:var varname] (self:reference-variable analysis varname name icolumn)
                                   [:const val] (add-clause analysis [:= [:column name icolumn] [:const val]])
                                   _ (error (.. "expected var or const, got " (fv value))))))
    _ (error (.. "Expected literal but got " (fv literal)))))

(fn new-analysis [] {:referenced-rules [] :variables [] :variable-mapping {} :selection [] :clauses [] :tables [] :constants []})

(fn Sqlog.gen-expr [self analysis expr]
  (match expr
    [:const val] (do (table.insert analysis.constants val) "?")
    [:column name icolumn] (.. name "." (match (. self.tables name)
                                          colnames (. colnames icolumn)
                                          _ (.. "c" icolumn)))
    [:as subexpr name] (.. (self:gen-expr analysis subexpr) " AS " name)
    [:= lhs rhs] (.. (self:gen-expr analysis lhs) " = " (self:gen-expr analysis rhs))
    _ (error (.. "Unrecognized expression " (fv expr)))))

(fn cat [list sep ?f]
  (table.concat (icollect [i v (ipairs list)] ((or ?f #$1) v i)) sep))

(fn Sqlog.gen-select [self analysis]
  (.. "SELECT "
    (if (> (length analysis.selection) 0)
        (cat analysis.selection ", " #(self:gen-expr analysis $1))
      "true")
    " FROM "
    (cat (lume.concat analysis.tables analysis.referenced-rules) " JOIN ")
    (if (> (length analysis.clauses) 0)
        (.. " WHERE " (cat analysis.clauses " AND " #(self:gen-expr analysis $1)))
      "")))

(fn Sqlog.query [self ...]
  (let [analysis (new-analysis)]
    (each [_ literal (ipairs [...])] (self:analyze-literal analysis literal))
    (set analysis.selection (icollect [_ varname (ipairs analysis.variables)] [:as (. analysis.variable-mapping varname) varname]))
    [(self:gen-select analysis) analysis.constants]))

Sqlog