(* A server-rendered SVG line chart with two data series on independent axes:
   weight on the left, reps on the right. No client script and no charting
   dependency -- the data is small and the shape is simple, so static SVG built
   from dream-html nodes fits the app's no-JS-first design and stays testable.

   SVG elements are built with [Dream_html.std_tag]; SVG in HTML5 needs no
   explicit namespace. Coordinates are plain floats in a fixed viewBox, so the
   chart scales to its container. *)

open Dream_html

let svg = std_tag "svg"
let polyline = std_tag "polyline"
let line = std_tag "line"
let text = std_tag "text"
let sattr = string_attr
let fattr = float_attr

(* Fixed drawing surface. The viewBox scales to the container width via CSS. A
   golden rectangle: the 640×396 surface is a 1.618:1 width:height ratio, which
   also keeps sparse two-point series from looking empty. The side gutters are
   fixed, so every chart's plot area starts at the same x and the stacked charts
   share one clean left edge. *)
let width = 640.
let height = 396.
let pad_left = 52.
let pad_right = 52.
let pad_top = 30.
let pad_bottom = 34.
let plot_w = width -. pad_left -. pad_right
let plot_h = height -. pad_top -. pad_bottom

(* Map a value in [lo, hi] to a y pixel, inverted so larger is higher. A flat
   series (lo = hi) pins to the vertical middle rather than dividing by zero. *)
let scale_y ~lo ~hi v =
  if Float.equal lo hi then pad_top +. (plot_h /. 2.)
  else pad_top +. (plot_h *. (1. -. ((v -. lo) /. (hi -. lo))))

(* Evenly spaced x for [n] points across the plot; a single point sits centre. *)
let scale_x ~n i =
  if n <= 1 then pad_left +. (plot_w /. 2.)
  else pad_left +. (plot_w *. float_of_int i /. float_of_int (n - 1))

let bounds values =
  match values with
  | [] -> (0., 1.)
  | first :: _ ->
      let lo = List.fold_left Float.min first values in
      let hi = List.fold_left Float.max first values in
      (* Pad a flat or tiny range so the line is not glued to an edge. *)
      if Float.equal lo hi then (lo -. 1., hi +. 1.) else (lo, hi)

let points_attr coords =
  coords
  |> List.map (fun (x, y) -> Printf.sprintf "%.1f,%.1f" x y)
  |> String.concat " "

(* A plain stroked line: no markers, no area fill. *)
let series ~css_class coords =
  polyline
    [
      sattr "class" "%s" css_class;
      sattr "fill" "none";
      sattr "points" "%s" (points_attr coords);
    ]
    []

let axis_line ~x1 ~y1 ~x2 ~y2 =
  line
    [
      sattr "class" "chart-axis";
      fattr "x1" x1;
      fattr "y1" y1;
      fattr "x2" x2;
      fattr "y2" y2;
    ]
    []

(* A faint full-width horizontal rule at fraction [f] of the plot height, to
   anchor the lines without competing with them. *)
let gridline f =
  let y = pad_top +. (plot_h *. f) in
  line
    [
      sattr "class" "chart-grid";
      fattr "x1" pad_left;
      fattr "y1" y;
      fattr "x2" (pad_left +. plot_w);
      fattr "y2" y;
    ]
    []

let axis_label ~x ~y ~anchor ~css_class s =
  text
    [
      sattr "class" "%s" css_class;
      fattr "x" x;
      fattr "y" y;
      sattr "text-anchor" "%s" anchor;
    ]
    [ txt "%s" s ]

(* Render the chart for a series of (weight, reps) points in time order. The
   caller guarantees at least two points. *)
let render (chart : View_model.Progress_page.chart) =
  let points = chart.points in
  let n = List.length points in
  let weights =
    List.map (fun (p : View_model.Progress_page.point) -> p.weight) points
  in
  let reps =
    List.map
      (fun (p : View_model.Progress_page.point) -> float_of_int p.reps)
      points
  in
  let w_lo, w_hi = bounds weights in
  let r_lo, r_hi = bounds reps in
  let weight_coords =
    List.mapi
      (fun i (p : View_model.Progress_page.point) ->
        (scale_x ~n i, scale_y ~lo:w_lo ~hi:w_hi p.weight))
      points
  in
  let rep_coords =
    List.mapi
      (fun i (p : View_model.Progress_page.point) ->
        (scale_x ~n i, scale_y ~lo:r_lo ~hi:r_hi (float_of_int p.reps)))
      points
  in
  svg
    [
      sattr "class" "progress-chart";
      sattr "viewBox" "0 0 %g %g" width height;
      sattr "role" "img";
      sattr "aria-label" "Line chart of weight and reps over time for %s"
        chart.exercise_name;
      sattr "preserveAspectRatio" "xMidYMid meet";
    ]
    (let first_date =
       match points with p :: _ -> p.View_model.Progress_page.date | [] -> ""
     in
     let last_date =
       match List.rev points with
       | p :: _ -> p.View_model.Progress_page.date
       | [] -> ""
     in
     [
       (* Faint gridlines first, so axes and series sit on top. *)
       gridline 0.25;
       gridline 0.5;
       gridline 0.75;
       (* Axes: left (weight), right (reps), and a baseline. *)
       axis_line ~x1:pad_left ~y1:pad_top ~x2:pad_left ~y2:(pad_top +. plot_h);
       axis_line ~x1:(pad_left +. plot_w) ~y1:pad_top ~x2:(pad_left +. plot_w)
         ~y2:(pad_top +. plot_h);
       axis_line ~x1:pad_left ~y1:(pad_top +. plot_h) ~x2:(pad_left +. plot_w)
         ~y2:(pad_top +. plot_h);
       (* Axis titles, anchored just above their axis, clear of the top edge. *)
       axis_label ~x:pad_left ~y:(pad_top -. 10.) ~anchor:"middle"
         ~css_class:"chart-axis-title weight" "kg";
       axis_label ~x:(pad_left +. plot_w) ~y:(pad_top -. 10.) ~anchor:"middle"
         ~css_class:"chart-axis-title reps" "reps";
       (* Left axis bounds (weight, kg). *)
       axis_label ~x:(pad_left -. 6.) ~y:(pad_top +. 4.) ~anchor:"end"
         ~css_class:"chart-tick weight" (Printf.sprintf "%g" w_hi);
       axis_label ~x:(pad_left -. 6.) ~y:(pad_top +. plot_h) ~anchor:"end"
         ~css_class:"chart-tick weight" (Printf.sprintf "%g" w_lo);
       (* Right axis bounds (reps). *)
       axis_label
         ~x:(pad_left +. plot_w +. 6.)
         ~y:(pad_top +. 4.) ~anchor:"start" ~css_class:"chart-tick reps"
         (Printf.sprintf "%g" r_hi);
       axis_label
         ~x:(pad_left +. plot_w +. 6.)
         ~y:(pad_top +. plot_h) ~anchor:"start" ~css_class:"chart-tick reps"
         (Printf.sprintf "%g" r_lo);
       (* X-axis: first and last dates. *)
       axis_label ~x:pad_left
         ~y:(pad_top +. plot_h +. 20.)
         ~anchor:"start" ~css_class:"chart-tick date" first_date;
       axis_label ~x:(pad_left +. plot_w)
         ~y:(pad_top +. plot_h +. 20.)
         ~anchor:"end" ~css_class:"chart-tick date" last_date;
       (* The two series. *)
       series ~css_class:"chart-series weight" weight_coords;
       series ~css_class:"chart-series reps" rep_coords;
     ])