groot: add write support for Branch+buffer of leaves

[sbinet]

use case "discovered" in:

• https://root-forum.cern.ch/t/converting-phasespace-to-root-file/47750

the following PyROOT script:

import numpy as np

dt = np.dtype([
    ('x', "f4"), ('y', "f4"), ("z", "f4"), 
    ("px", "f4"), ("py", "f4"), 
    ("Ekin", "f4"), ("wepl", "f4"), 
    ("pdgID", "<i4"), 
    ("flag", "<u2"), 
    ("tof", "f4"),
    ("runID","<i4"),
    ("evtID","<i4"),
    ("trkID","<i4"),
    ("parID","<i4")])

evts = np.fromfile("./PhaseSpace.phsp", dtype=dt)
evts = np.sort(evts, order="evtID")

import ROOT
f = ROOT.TFile.Open("out.root", "RECREATE")
t = ROOT.TTree("PhaseSpaceTree", "PhaseSpaceTree")

dt = np.dtype([
    ('x', "f4"), ('y', "f4"), ("z", "f4"), 
    ("px", "f4"), ("py", "f4"), ("pz", "f4"),
    ("Ekin", "f4"), ("wepl", "f4"), 
    ("pdgID", "<i4"), 
    ("event", "<i4"),
    ("trackID","<i4"),
    ("PB_ID","<i4")])

buf = np.array([
    0,0,0,
    0,0,0,
    0,0,
    0,0,0
    ], dtype=dt)

t.Branch("PhaseSpaceBranch", buf, "x/F:y/F:z/F:px/F:py/F:pz/F:Ekin/F:wepl/F:pdgID/I:event/I:trackID/I:PB_ID/I")
for v in evts:
    buf["x"] = v["x"]
    buf["y"] = v["y"]
    buf["z"] = v["z"]
    buf["px"] = v["px"]
    buf["py"] = v["py"]
    buf["pz"] = 1 - buf["px"]**2 - buf["py"]**2
    buf["Ekin"] = v["Ekin"]
    buf["wepl"] = v["wepl"]
    buf["pdgID"] = v["pdgID"]
    buf["event"] = v["evtID"]
    buf["trackID"] = v["trkID"]
    buf["PB_ID"] = v["parID"]
    t.Fill()
t.Write()
f.Close()

will create a ROOT file+tree with the following structure:

$> root-ls -t ./out.root 
=== [./out.root] ===
version: 62406
  TTree              PhaseSpaceTree                                       PhaseSpaceTree (entries=4122)
    PhaseSpaceBranch "x/F:y/F:z/F:px/F:py/F:pz/F:Ekin/F:wepl/F:pdgI[...]" TBranch

$> root-dump ./out.root | head -n 20
>>> file[./out.root]
key[000]: PhaseSpaceTree;1 "PhaseSpaceTree" (TTree)
[000][PhaseSpaceBranch.x]: -1.95
[000][PhaseSpaceBranch.y]: 0.18836617
[000][PhaseSpaceBranch.z]: -0.23779297
[000][PhaseSpaceBranch.px]: 0.99966717
[000][PhaseSpaceBranch.py]: -0.019618336
[000][PhaseSpaceBranch.pz]: 0.00028066634
[000][PhaseSpaceBranch.Ekin]: 86.00329
[000][PhaseSpaceBranch.wepl]: 1
[000][PhaseSpaceBranch.pdgID]: 2212
[000][PhaseSpaceBranch.event]: 0
[000][PhaseSpaceBranch.trackID]: 1
[000][PhaseSpaceBranch.PB_ID]: 999
[001][PhaseSpaceBranch.x]: -1.95
[001][PhaseSpaceBranch.y]: -0.44054914
[001][PhaseSpaceBranch.z]: 0.03474723
[001][PhaseSpaceBranch.px]: 0.9999974
[001][PhaseSpaceBranch.py]: 0.0012126022
[001][PhaseSpaceBranch.pz]: 3.7748046e-06

right now (v0.29), groot/rtree doesn't provide the ability to fill a Tree from a contiguous buffer of bytes, with some metadata to instruct on how to interpret that buffer.

• testdata.zip