;;; A metric induces a torsion-free connection			       
	       
(define (metric->Christoffel-1 metric basis)
  (let ((vector-basis (basis->vector-basis basis)))
    (make-Christoffel
     (s:map/r (lambda (e_k)
		(s:map/r (lambda (e_j)
			   (s:map/r (lambda (e_i)
				      (* 1/2 (- (+ (e_k (metric e_i e_j))
						   (e_j (metric e_i e_k)))
						(e_i (metric e_j e_k)))))
				    vector-basis))
			 vector-basis))
	      vector-basis)
     basis)))

#|
(define 2-sphere R2-rect)
(install-coordinates 2-sphere (up 'theta 'phi))

(define ((g-sphere R) u v)
  (* (square R)
     (+ (* (dtheta u) (dtheta v))
	(* (compose (square sin) theta)
	   (dphi u)
	   (dphi v)))))

(pec ((Christoffel->symbols
       (metric->Christoffel-1 (g-sphere 'R)
			      (coordinate-system->basis 2-sphere)))
      ((2-sphere '->point) (up 'theta0 'phi0))))
#| Result:
(down
 (down (down 0 0) (down 0 (* (* (cos theta0) (sin theta0)) (expt R 2))))
 (down (down 0 (* (* (cos theta0) (sin theta0)) (expt R 2)))
       (down (* (* -1 (cos theta0) (sin theta0)) (expt R 2)) 0)))
|#
|#

#|
;;; Check of text procedure for getting second Christoffel symbols

(define (metric->Christoffel-2 metric basis)
  (let ((gi (metric:invert metric basis))
	(G1 (metric->Christoffel-1 metric basis)))
    (let ((vector-basis (basis->vector-basis basis))
	  (1form-basis (basis->1form-basis basis))
	  (G1S (Christoffel->symbols G1)))
      (define ((Gamma-Bar v w) u)
	(let ((stuff
	       (s:map/r (lambda (e_k)
			  (s:map/r (lambda (e_j)
				     (s:map/r (lambda (e_i)
						(* (e_i u) (e_j v) (e_k w)))
					      1form-basis))
				   1form-basis))
			1form-basis)))
	  (apply + (ultra-flatten (s:map/r * G1S stuff)))))
      (define (Gamma-hat v w)
	(apply +
	       (ultra-flatten
		(s:map/r
		 (lambda (e~i e_i)
		   (* (gi (Gamma-Bar v w) e~i) e_i))
		 1form-basis vector-basis))))
      (make-Christoffel
       (s:map/r (lambda (e_k)
		  (s:map/r (lambda (e_j)
			     (s:map/r (lambda (e~i)
					(e~i (Gamma-hat e_j e_k)))
				      1form-basis))
			   vector-basis))
		vector-basis)       
       basis))))


(pec ((Christoffel->symbols
       (metric->Christoffel-2 (g-sphere 'R)
			      (coordinate-system->basis 2-sphere)))
      ((2-sphere '->point) (up 'theta0 'phi0))))
#| Result:
(down (down (up 0 0)
	    (up 0 (/ (cos theta0) (sin theta0))))
      (down (up 0 (/ (cos theta0) (sin theta0)))
	    (up (* -1 (cos theta0) (sin theta0)) 0)))
|#
;;; As expected!
|#

#|
;;; Test with general 2d metric

(install-coordinates R2-rect (up 'x 'y))

(define fa
  (compose (literal-function 'a (-> (UP Real Real) Real))
	   (R2-rect '->coords)))
(define fb
  (compose (literal-function 'b (-> (UP Real Real) Real))
	   (R2-rect '->coords)))
(define fc
  (compose (literal-function 'c (-> (UP Real Real) Real))
	   (R2-rect '->coords)))

(define ((g-R2 g_00 g_01 g_11) u v)
  (+ (* g_00 (dx u) (dx v))
     (* g_01 (+ (* (dx u) (dy v)) (* (dy u) (dx v))))
     (* g_11 (dy u) (dy v))))

(pec (((g-R2 fa fb fc)
       (literal-vector-field 'u R2-rect)
       (literal-vector-field 'v R2-rect))
      ((R2-rect '->point) (up 'x0 'y0))))
#| Result:
(+ (* (v^1 (up x0 y0)) (u^1 (up x0 y0)) (c (up x0 y0)))
   (* (v^0 (up x0 y0)) (b (up x0 y0)) (u^1 (up x0 y0)))
   (* (u^0 (up x0 y0)) (b (up x0 y0)) (v^1 (up x0 y0)))
   (* (a (up x0 y0)) (u^0 (up x0 y0)) (v^0 (up x0 y0))))
|#

(define R2-basis (coordinate-system->basis R2-rect))

(pec ((Christoffel->symbols
       (metric->Christoffel-1 (g-R2 fa fb fc) R2-basis))
      ((R2-rect '->point) (up 'x0 'y0))))
#| Result:
(down
 (down
  (down (* 1/2 (((partial 0) a) (up x0 y0)))
	(+ (* -1/2 (((partial 1) a) (up x0 y0)))
	   (((partial 0) b) (up x0 y0))))
  (down (* 1/2 (((partial 1) a) (up x0 y0)))
        (* 1/2 (((partial 0) c) (up x0 y0)))))
 (down
  (down (* 1/2 (((partial 1) a) (up x0 y0)))
        (* 1/2 (((partial 0) c) (up x0 y0))))
  (down (+ (((partial 1) b) (up x0 y0))
	   (* -1/2 (((partial 0) c) (up x0 y0))))
        (* 1/2 (((partial 1) c) (up x0 y0))))))
|#
|#

(define (metric->Christoffel-2 metric basis)
  (let ((gi (metric:invert metric basis)))
    (let ((vector-basis (basis->vector-basis basis))
	  (1form-basis (basis->1form-basis basis)))
      (make-Christoffel
       (s:map/r (lambda (e_k)
		  (s:map/r (lambda (e_j)
			     (s:map/r (lambda (w_i)
					(contract-2
					 (lambda (e_m w_m)
					   (* (gi w_i w_m)
					      (* 1/2
						 (- (+ (e_k (metric e_m e_j))
						       (e_j (metric e_m e_k)))
						    (e_m (metric e_j e_k))))))
					 basis))
				      1form-basis))
			   vector-basis))
		vector-basis)
       basis))))

#|
(pec ((Christoffel->symbols
       (metric->Christoffel-2 (g-sphere 'R)
			      (coordinate-system->basis 2-sphere)))
      ((2-sphere '->point) (up 'theta0 'phi0))))
#| Result:
(down
 (down (up 0 0) (up 0 (/ (cos theta0) (sin theta0))))
 (down (up 0 (/ (cos theta0) (sin theta0)))
       (up (* -1 (sin theta0) (cos theta0)) 0)))
|#
|#

;;; Symbolic metrics are often useful for testing.

(define (make-metric coordinate-system)
  (define (gij i j)
    (if (<= i j)
	(literal-manifold-function
	 (string->symbol
	  (string-append "g"
			 "_"
			 (number->string i)
			 (number->string j)))
	 coordinate-system)
	(gij j i)))
  gij)
				    
(define (literal-metric coordinate-system)
  ;; Flat coordinate systems here only.
  (let ((basis (coordinate-system->basis coordinate-system)))
    (let ((1form-basis (basis->1form-basis basis))
	  (gij (make-metric coordinate-system)))
      (let ((n (s:dimension 1form-basis)))
	(let ((gcoeffs
	       (s:generate n 'down
			   (lambda (i)
			     (s:generate n 'down
					 (lambda (j)
					   (gij i j)))))))
	  (lambda (v1 v2)
	    (* (* gcoeffs (1form-basis v1))
	       (1form-basis v2))))))))
#|
(install-coordinates R3-rect (up 'x 'y 'z))

(set! *factoring* #f)

(pec (((literal-metric R3-rect)
       (literal-vector-field 'u R3-rect)
       (literal-vector-field 'v R3-rect))
      ((R3-rect '->point) (up 'x0 'y0 'z0))))
#| Result:
(+ (* (v^0 (up x0 y0 z0)) (u^0 (up x0 y0 z0)) (g_00 (up x0 y0 z0)))
   (* (v^0 (up x0 y0 z0)) (g_01 (up x0 y0 z0)) (u^1 (up x0 y0 z0)))
   (* (v^0 (up x0 y0 z0)) (g_02 (up x0 y0 z0)) (u^2 (up x0 y0 z0)))
   (* (u^0 (up x0 y0 z0)) (v^1 (up x0 y0 z0)) (g_01 (up x0 y0 z0)))
   (* (u^0 (up x0 y0 z0)) (v^2 (up x0 y0 z0)) (g_02 (up x0 y0 z0)))
   (* (v^1 (up x0 y0 z0)) (u^1 (up x0 y0 z0)) (g_11 (up x0 y0 z0)))
   (* (v^1 (up x0 y0 z0)) (g_12 (up x0 y0 z0)) (u^2 (up x0 y0 z0)))
   (* (v^2 (up x0 y0 z0)) (u^1 (up x0 y0 z0)) (g_12 (up x0 y0 z0)))
   (* (v^2 (up x0 y0 z0)) (u^2 (up x0 y0 z0)) (g_22 (up x0 y0 z0))))
|#
|#

#|
(define polar R2-polar)

(install-coordinates polar (up 'r 'theta))

(define polar-point 
  ((polar '->point) (up 'r 'theta)))

(define polar-basis
  (coordinate-system->basis polar))

(define (polar-metric v1 v2)
  (+ (* (dr v1) (dr v2))
     (* (square r)
	(* (dtheta v1) (dtheta v2)))))

(define foo
  ((Christoffel->symbols
    (metric->Christoffel-2 polar-metric polar-basis))
   polar-point))

(pec foo)
#| Result:
(down
 (down (up 0 0)
       (up 0 (/ 1 r)))
 (down (up 0 (/ 1 r))
       (up (* -1 r) 0)))
|#

;;; Faster, a simplified version.

(define polar R2-rect)

(install-coordinates polar (up 'r 'theta))

(define polar-point 
  ((polar '->point) (up 'r 'theta)))

(define polar-Gamma
  (make-Christoffel
   (let ((O (lambda x 0)))
     (down
      (down (up O O)
	    (up O (/ 1 r)))
      (down (up O (/ 1 r))
	    (up (* -1 r) O))))
   (coordinate-system->basis polar)))

;;; Now look at curvature
(let ((curvature (Riemann (Christoffel->Cartan polar-Gamma))))
  (for-each
   (lambda (alpha)
     (for-each
      (lambda (beta)
	(for-each
	 (lambda (gamma)
	   (for-each
	    (lambda (delta)
	      (newline)
	      (pe `(,alpha ,beta ,gamma ,delta))
	      (pe ((curvature alpha beta gamma delta) polar-point)))
	    (list d/dr d/dtheta)))
	 (list d/dr d/dtheta)))
      (list d/dr d/dtheta)))
   (list dr dtheta)))
;;; 16 zeros
|#

#|
(define spherical R3-rect)
(install-coordinates spherical (up 'r 'theta 'phi))

(define spherical-point 
  ((spherical '->point) (up 'r 'theta 'phi)))

(define spherical-basis
  (coordinate-system->basis spherical))

(define (spherical-metric v1 v2)
  (+ (* (dr v1) (dr v2))
     (* (square r)
	(+ (* (dtheta v1) (dtheta v2))
	   (* (expt (sin theta) 2)
	      (dphi v1) (dphi v2))))))

(define foo
  ((Christoffel->symbols
    (metric->Christoffel-2 spherical-metric spherical-basis))
   spherical-point))

(pec foo)
#| Result:
(down
 (down (up 0 0 0) (up 0 (/ 1 r) 0) (up 0 0 (/ 1 r)))
 (down (up 0 (/ 1 r) 0) (up (* -1 r) 0 0) (up 0 0 (/ (cos theta) (sin theta))))
 (down (up 0 0 (/ 1 r))
       (up 0 0 (/ (cos theta) (sin theta)))
       (up (* -1 r (expt (sin theta) 2)) (* -1 (sin theta) (cos theta)) 0)))
|#

;;; Thus, make simplified version.

(define spherical-Gamma
  (make-Christoffel
   (let ((O (lambda x 0)))
     (down
      (down (up O O O) (up O (/ 1 r) O) (up O O (/ 1 r)))
      (down (up O (/ 1 r) O) (up (* -1 r) O O) (up O O (/ (cos theta) (sin theta))))
      (down (up O O (/ 1 r))
	    (up O O (/ (cos theta) (sin theta)))
	    (up (* -1 r (expt (sin theta) 2)) (* -1 (sin theta) (cos theta)) O))))
   (coordinate-system->basis spherical)))

;;; Now look at curvature
(let ((curvature (Riemann (Christoffel->Cartan spherical-Gamma))))
  (for-each
   (lambda (alpha)
     (for-each
      (lambda (beta)
	(for-each
	 (lambda (gamma)
	   (for-each
	    (lambda (delta)
	      (newline)
	      (pe `(,alpha ,beta ,gamma ,delta))
	      (pe ((curvature alpha beta gamma delta)
		   spherical-point)))
	    (list d/dr d/dtheta d/dphi)))
	 (list d/dr d/dtheta d/dphi)))
      (list d/dr d/dtheta d/dphi)))
   (list dr dtheta dphi)))
;;; 81 zeros
|#

;;; Connections for non-coordinate basis -- MTW p.210

;;; c_ijk = g_kl c_ij^l = g_kl e^l([e_i, e_j])

(define (structure-constant e_i e_j e_k basis metric)
  (contract-2 
   (lambda (e_l w_l)
     (* (metric e_k e_l)
	(w_l (commutator e_i e_j))))
   basis))

(define (metric->connection-1 metric basis)
  (let ((vector-basis (basis->vector-basis basis))
	(1form-basis (basis->1form-basis basis)))
    (make-Christoffel
     (s:map/r
      (lambda (e_k)
	(s:map/r
	 (lambda (e_j)
	   (s:map/r
	    (lambda (e_i)
	      (* 1/2 (+ (- (+ (e_k (metric e_i e_j))
			      (e_j (metric e_i e_k)))
			   (e_i (metric e_j e_k)))
			(- (+ (structure-constant e_i e_j e_k basis metric)
			      (structure-constant e_i e_k e_j basis metric))
			   (structure-constant e_j e_k e_i basis metric)))))
	    vector-basis))
	 vector-basis))
      vector-basis)
     basis)))


(define (metric->connection-2 metric basis)
  (let ((vector-basis (basis->vector-basis basis))
	(1form-basis (basis->1form-basis basis))
	(inverse-metric (metric:invert metric basis)))
    (make-Christoffel
     (s:map/r
      (lambda (e_k)
	(s:map/r
	 (lambda (e_j)
	   (s:map/r
	    (lambda (w_i)
	      (contract-2
	       (lambda (e_m w_m)
		 (* (inverse-metric w_i w_m)
		    (* 1/2 (+ (- (+ (e_k (metric e_m e_j))
				    (e_j (metric e_m e_k)))
				 (e_m (metric e_j e_k)))
			      (- (+ (structure-constant e_m e_j e_k basis metric)
				    (structure-constant e_m e_k e_j basis metric))
				 (structure-constant e_j e_k e_m basis metric))))))
	       basis))
	    1form-basis))
	 vector-basis))
      vector-basis)
     basis)))

#|
;;; MTW p205 spherical flat lorentz

(define spherical-Lorentz R4-rect)
(install-coordinates spherical-Lorentz (up 't 'r 'theta 'phi))

(define spherical-Lorentz-basis
  (coordinate-system->basis spherical-Lorentz))

(define ((spherical-Lorentz-metric c^2) v1 v2)
  (+ (* -1 c^2 (* (dt v1) (dt v2)))
     (* (dr v1) (dr v2))
     (* (square r)
	(+ (* (dtheta v1) (dtheta v2))
	   (* (square (sin theta))
	      (* (dphi v1) (dphi v2)))))))

(define spherical-Lorentz-point 
  ((spherical-Lorentz '->point) (up 't 'r 'theta 'phi)))

(define (orthonormal-spherical-Lorentz-vector-basis c^2)
  (down (* (/ 1 (sqrt c^2)) d/dt)
	d/dr
	(* (/ 1 r) d/dtheta)
	(* (/ 1 (* r (sin theta))) d/dphi)))

(define (orthonormal-spherical-Lorentz-1form-basis c^2)
  (let ((orthonormal-spherical-Lorentz-vectors
	 (orthonormal-spherical-Lorentz-vector-basis c^2)))
    (vector-basis->dual orthonormal-spherical-Lorentz-vectors 
			spherical-Lorentz)))

(define (orthonormal-spherical-Lorentz-basis c^2)
  (make-basis (orthonormal-spherical-Lorentz-vector-basis c^2)
	      (orthonormal-spherical-Lorentz-1form-basis c^2)))

(pec ((s:map/r (orthonormal-spherical-Lorentz-1form-basis 'c^2)
	       (orthonormal-spherical-Lorentz-vector-basis 'c^2))
      spherical-Lorentz-point))
#| Result:
(down (up 1 0 0 0) (up 0 1 0 0) (up 0 0 1 0) (up 0 0 0 1))
|#

(pec (((spherical-Lorentz-metric 'c^2)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 0)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 0))
      spherical-Lorentz-point))
#| Result:
-1
|#

(pec (((spherical-Lorentz-metric 'c^2)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 1)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 1))
      spherical-Lorentz-point))
#| Result:
1
|#

(pec (((spherical-Lorentz-metric 'c^2)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 2)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 2))
      spherical-Lorentz-point))
#| Result:
1
|#

(pec (((spherical-Lorentz-metric 'c^2)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 3)
       (ref (orthonormal-spherical-Lorentz-vector-basis 'c^2) 3))
      spherical-Lorentz-point))
#| Result:
1
|#

(pec ((Christoffel->symbols
       (metric->connection-1 (spherical-Lorentz-metric 'c^2)
			     (orthonormal-spherical-Lorentz-basis 'c^2)))
      spherical-Lorentz-point))
#| Result:
(down
 (down (down 0 0 0 0) (down 0 0 0 0) (down 0 0 0 0) (down 0 0 0 0))
 (down (down 0 0 0 0) (down 0 0 0 0) (down 0 0 0 0) (down 0 0 0 0))
 (down (down 0 0 0 0) (down 0 0 (/ 1 r) 0) (down 0 (/ -1 r) 0 0) (down 0 0 0 0))
 (down (down 0 0 0 0)
       (down 0 0 0 (/ 1 r))
       (down 0 0 0 (/ (cos theta) (* r (sin theta))))
       (down 0 (/ -1 r) (/ (* -1 (cos theta)) (* r (sin theta))) 0)))
|#

(define foo
  (show-time
   (lambda ()
     ((Christoffel->symbols
       (metric->connection-2 (spherical-Lorentz-metric 'c^2)
			     (orthonormal-spherical-Lorentz-basis 'c^2)))
      spherical-Lorentz-point))))

(pec foo)
#| Result:
(down
 (down (up 0 0 0 0) (up 0 0 0 0) (up 0 0 0 0) (up 0 0 0 0))
 (down (up 0 0 0 0) (up 0 0 0 0) (up 0 0 0 0) (up 0 0 0 0))
 (down (up 0 0 0 0) (up 0 0 (/ 1 r) 0) (up 0 (/ -1 r) 0 0) (up 0 0 0 0))
 (down (up 0 0 0 0)
       (up 0 0 0 (/ 1 r))
       (up 0 0 0 (/ (cos theta) (* r (sin theta))))
       (up 0 (/ -1 r) (/ (* -1 (cos theta)) (* r (sin theta))) 0)))
|#
;;; The last two are essentially the same.  Is this correct?

#|
;;; Check answers from MTW p.213
;;; t r theta phi
;;; 0 1 2     3

(pe (ref foo 3 2 3))
(/ (cos theta) (* r (sin theta)))

(pe (ref foo 3 3 2))
(/ (* -1 (cos theta)) (* r (sin theta)))

(pe (ref foo 2 1 2))
(/ 1 r)

(pe (ref foo 3 1 3))
(/ 1 r)

(pe (ref foo 2 2 1))
(/ -1 r)

(pe (ref foo 3 3 1))
(/ -1 r)
|#

(define (orthonormal-spherical-Lorentz-second-connection c^2)
  (make-Christoffel
   (let ((zero (lambda (point) 0)))
     (down
      (down (up zero zero zero zero)
	    (up zero zero zero zero)
	    (up zero zero zero zero)
	    (up zero zero zero zero))
      (down (up zero zero zero zero)
	    (up zero zero zero zero)
	    (up zero zero zero zero)
	    (up zero zero zero zero))
      (down (up zero zero zero zero)
	    (up zero zero (/ 1 r) zero)
	    (up zero (/ -1 r) zero zero)
	    (up zero zero zero zero))
      (down (up zero zero zero zero)
	    (up zero zero zero (/ 1 r))
	    (up zero zero zero (/ (cos theta) (* r (sin theta))))
	    (up zero
		(/ -1 r)
		(/ (* -1 (cos theta)) (* r (sin theta)))
		zero))))
   (orthonormal-spherical-Lorentz-basis c^2)))

;;; Look at curvature
(for-each
 (lambda (alpha)
   (for-each
    (lambda (beta)
      (for-each
       (lambda (gamma)
	 (for-each
	  (lambda (delta)
	    (newline)
	    (pe `(,alpha ,beta ,gamma ,delta))
	    (pe (((Riemann
		   (Christoffel->Cartan
		    (orthonormal-spherical-Lorentz-second-connection 'c^2)))
		  alpha beta gamma delta)
		 spherical-Lorentz-point)))
	  (list d/dt d/dr d/dtheta d/dphi)))
       (list d/dt d/dr d/dtheta d/dphi)))
    (list d/dt d/dr d/dtheta d/dphi)))
 (list dt dr dtheta dphi))
;;; 256 zeros
|#